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Temporary Power Distribution webinar

Your questions answered

On a building site, does the metal scaffolding need to be earthed?

It depends on the application. If the scaffolding has electrical equipment in use or installed on it, it may constitute an extraneous-conductive part and so will need protective bonding as required by BS 7671.

This will need to be coordinated with any lightning protection requirements. See also SG5:18. Overhead Power Sources and Earthing of Scaffold Structures from

EV charging: A single generator has an (IN-S) earthing arrangement with its output protected by a 100mA RCD with a disconnection time of less than 0.2sec. The generator is supplying multiple EV charging stations (with PEN protection & type B RCD) in close proximity (5m apart), however the EV chargers are not independently earthed, the only connection to earth they have is Mass of earth and the CPC of the final circuit. would this be considered good working practice? Same question for (TN-S) earthing of a generator?

Without more information, it is not possible to give a definitive answer to this question. The key point is that being IN-S or TN-S, there is no PEN conductor in the system and therefore the risks associated with neutral failure are not present. On that basis a connection with the mass of Earth may not be required and the PEN protection is unnecessary.

When power to a site is placed on a generator due to transformer failure at what point would a temporary setup be classed as permanent?

If the intention is to return the site back to grid power, then it will be temporary for as long as the generator powers the installation. There is no limit to how long it may last as a temporary system, so long as the intention is to remove the generator when grid power becomes available.

What is the procedure for sizing a temporary generator? What inputs are required to calculate the size required, what power factors should be used? What happens if it is over or undersized?

There are two issues to be considered here. Firstly you need to make a reasoned estimate of the load current (which of course may not be accurately known until everything is connected). Take into account start-up currents for connected loads, such as large motors. Verify if the generator is a stand-by set or a prime power one; stand-by units typically do not have any overload ability.

Secondly estimate the combined load power factor, erring on the side of caution. Generator ratings are usually specified at a load PF of 0.8. So for example, a 100 kVA generator is rated to deliver 100 kVA into a load with a PF of 0.8, i.e. an 80 kW load. The key thing to note is that the engine is likely to only have a maximum power output of a little over this figure, say 90 kW. Most generators will not deliver their rated kVA as kW into a load with a PF approaching 1. So a 100 kVA generator will not power a load of 100 kW, and that is a risk of an undersized generator. Beware of leading power factors as this is likely to lead to generator instability. A resistive loadbank may be used to alleviate this problem.

Oversizing can result in inefficiency and engine issues. Also consider harmonics - many alternator manufacturers require de-rating (by as much as 50%) of the rated output when powering loads generating low-order harmonics.

When you’re using a generator to supply TV lighting inside a building for example, it’s good practice to link the generator Earth to the building MET, as per section 9.2 in the Practitioner's guide to Temporary Power Systems.

If you can’t get to the MET inside the building to make that link - what are your options? You can access 13A twin sockets, and radiators only - Is it worth connecting to either of these and what should you test before making that connection?

This has to be considered on an individual basis (e.g. accessibility of equipment, use of the building supply etc). In essence, the idea is to maintain the equipotential zone if there is more than one supply present. If it does need joining, try and do so at the main earthing terminal. If this is not possible, try and identify extraneous-conductive parts on the building that are already bonded and use those. Utilising the protective conductor in a socket may be suitable (if practical to do so), but consider the sizing of the conductors under fault conditions.

What are the key standards for up to 1kV?

The same as for 230/400 V, i.e. BS 7671/7909 as required.

Do you know what the changes are going to be with part 551 of BS 7671?

We cannot divulge the detail of discussions in Standards committees, but in essence given the growth of prosumer installations and the work done on temporary generators generally, there is need for a wholesale review of section 551. This is underway at an international level with the IEC, so it will be some years before it filters down via CENELEC and be implemented in BS 7671.

BS 7909 is a CoP for Temporary Electrical Systems for entertainment and related purposes. Is there a similar CoP for temporary installs onboard Ships? or are there no defined rules for this type of work? If there is a CoP, what BS number would that be?

I am not aware of a CoP for temporary systems onboard ships. I would suggest that the technical principles of BS 8450:2006 Code of practice for the installation of electrical and electronic equipment in ships would be appropriate to adopt, noting that the installation methods/requirements may need adapting considering the temporary nature.

If a temporary piece of distribution has residual current protection with type AC characteristics feeding electronics, (switch mode PSU's etc.) and cannot be swapped out, would it be permissible to test the RCD at x1 and x5 whilst the electronics are on full load (where possible) to see if the RCD is being affected? Or is there a better way?

It is also possible to see if the 'T' button operates the device. If it is being defeated by DC components caused by the loads, then it *may* show up. However, because the load profile will change over time (e.g. batteries in laptops reaching full charge or equipment changing state such as printer switching form idle to printing), any test result would only be valid at the time it was conducted. If the device can't be swapped out, consider inserting another temporary distribution unit with Type A or B devices fitted inline with the Type AC.

Sometimes on longer circuits, 30mA earth leakage protection is is used as fault protection and we can have a maximum Zs of 1667ohms. This is fine if there is a L-E fault but what about a L-N fault. Is this something we should be worried about?

Yes. BS 7671 requires 'fault protection' which may be a fault to earth or a short-circuit - see the definition of 'fault' in Part 2. A short circuit between lines or line and live conductors may not pose a risk of shock. If so, the risk is typically one of thermal damage. The adiabatic equation can be used to check conductor sizes for disconnection times of five seconds or less. If over five seconds, refer to BS 7454.

Given the commitment for UK government for climate change and the movement away from fossil fuel, how do you see the future of temporary power requirements being met in a practical way (i.e. large lithium battery banks are very heavy and carry their own unique hazards!)?

Progress is being made in alternative technologies. Generators are typically deployed to provide small-scale reliable and continuous power generation, often at significant levels. Renewable technologies are not well-suited to that application. That said, methods such as hydrogen fuel cells are becoming viable at low power levels. Common efficiency methods include load-on-demand generator sequencing, the use of HVO and similar fuels as well as hybrid generator battery sets. It is though an area that needs investment to develop sustainable technologies.

Have you experience of practical solutions that can be applied to direct exhaust fumes away from a practically sited diesel generator that might present problems for people local to the site?

No. Generally we would site a generator to take into account environmental factors. The manufacturer should be consulted if exhaust extensions are considered.

Can you go over the generator earthing? Is this just a spike in the ground with each generator to get a resistance below 20 Ohm or do you need to do a full earth design for touch and step voltages etc.?

The value of 20 Ohms stems from HV networks in the national grid and for LV generators this figure does not apply. Step potentials and touch voltages are also generally not considered an issue at LV in a properly designed system. More information is available in the presentation.

Do we require life safety supplies for (wet riser etc.) on the temporary supplies?

If supplies are required for safety services then the general requirements apply (e.g. BS 7671 Chapters 35 & 56), regardless of whether the installation is temporary or not.

How can you account for a collective of nasty motor starting currents (i.e. a set of pumps or fans) if connected to a temporary generator (i.e. maybe 6 of 10 motors start together and give for a significant combined start current)? How far do you allow of current above that of normal running, allowing for some diversty?

There are several factors to consider here. Firstly is the generator a prime power or standby set? What starters do the motors use? If using VFDs will harmonics need compensating for? The ability of a generator to recover the output voltage from a significant load is a function of engine power (kW) and the alternator excitation ability which also varies. It is best to consult with the manufacturer having knowledge of the loads. As a very rough rule of thumb for sizing is that a motor starting current is about six times the running current.

Could you give any guidance for safely and practically using power from inside a building for items outside that building for a short duration. E.g. 3 or 4 Class 1 lights to film a short, exterior drama scene? I'm particularly wondering about the implications of exporting the earth.

The risk here is one of the failure of the neutral in the supply to the building, but only if the supply is TN-C-S (i.e. PME). If that happens, people outside would be at more risk of shock, being outside the equipotential zone of the building. For a short duration of a couple of hours the risk is very low and, following your risk assessment, may be considered negligible. If it cannot be verified that the supplies to the lights are RCD protected, then inline RCDs (or similar) should be used for additional protection, though note that this is for general shock protection and will not help in the event of a PEN failure.

Doesn't rubber insulated cable degrade with UV etc?

Modern RN or RR cables use a synthetic rubber compound with greater protection from UV damage.

What do the regulations say about removing ELCB protection from an installed temporary supply, to prevent it interfering with a directly downstream RCD on a toured system?

With the owners permission an installation may be modified, though such a modification would need to be designed, completed and tested in accordance with BS 7671. The voltage operated ELCB will not 'interfere' with the RCD, but if left in circuit may still be subject to unwanted operation.

Is it true that blue "arctic" cable is against regulations and as event installers we can refuse to power traders who bring (even if portable applicance tested) these cables? Or if the cable has passed all tests including on-site testing, would we have no grounds in refusing to use it?

No. As covered during the live Q&A, the use of such cable is not 'banned', but is not recommended for general outdoor use as it not suitable for rough environments. The cable can be used if the designer is satisfied that it is suitable for the installation conditions and not likely to suffer from (or is protected against) mechanical damage.

You stated that PVC cable can only be used if it has appropriate mechanical protection. Could you give an example of what this would be?

Suitable mechanical protection could be cable ramps, or routing it in a position where damage is unlikely, such as along a roof truss of a tent for example.

With regard to cable selection, what is the view on Radaflex which is being used by generator companies on lots of Film and TV jobs?

The designer is responsible for specifying the correct cable for the installation. BS 7671 requires cables to comply with *appropriate* harmonised European Standards or British Standards.

SY cables are really nice, they are obviously temporary and if correctly sized and when used within their ratings (A and C) are a good solution. Please expand your comment about SY cables.

SY cables are not suitable for temporary installations. Manufacturers data typically states that it is used for interconnections in measuring, controlling or regulation in production lines, conveyors and similar, or installations where only light mechanical stress may occur. It may be used outdoors if suitably protected (to quote another data sheet). The braid needs earthing in an appropriate gland (not possible in a typical connector) and the sheath is not often UV stable. Degradation of the clear sheath is common and usually leads to exposed braiding and subsequent rusting. The sheath is ordinary PVC and therefore subject to damage like any PVC cable, which is not recommended in BS 7671 or BS 7909 for outdoor rough use.

Furthermore, such cables do not comply with harmonised European Standards and therefore the designer of the installation must make a declaration that the cable meets the same requirements as a harmonised cable would. On a final note, one manufacturer of a good quality cable states on its data sheet that "SY cables are not suitable for direct connection to the public mains supply".

What's your thoughts on SPDs for temporary power, and also AFDDs as there is big talk about it in industry at the moment?

The designer of the temporary installation must understand the intent of the requirement for SPDs. They are to protect the installation from voltages from atmospheric or external sources (e.g. lightning) as well as switching from within the installation. If the supply is derived from generators, atmospheric effects are unlikely. If the installation contains no equipment likely to generate switching overvoltages such as large motors, then a risk assessment may decide SPDs can be omitted.

AFDDs are currently only recommended.

I use temporary generator sets when we have flooding and we drive in earth rods. Are earth rods still required if earth leakage device is fitted or should it always be used together?

See the presentation. It depends on the generator topology. If an RCD is fitted on the generator output that would have a bearing on the earth electrode resistance value, if one is required.

Is it a requirement to measure the resistance from the earth electrode to the mass of earth?

If the electrode is being used for fault detection, then yes. The value of the electrode resistance needs to be coordinated with the output protective device on the generator.

In regards to a temporary system, used in events for example, how do you decide on what/where a final circuit becomes final? Where does the responsibility for that circuit finish?

A final circuit is one which current-using equipment will be connected. So a socket-outlet into which a mixing desk or moving light is connected is a final circuit. A socket-outlet which is installed for others to connect their distribution equipment (e.g. a 400A TPN supply for lighting or a 63A socket for video) is a distribution circuit. Responsibility should be managed across site and is the aim of BS 7909. If you are, say, only installing power for others to connect to, then unless otherwise instructed by the production company, your responsibility stops at the end of your cables.

Where more than one generator is utilised on a site, is it necessary to interconnect the earthing systems or are there risks with this?

It is very unlikely to present a risk. As a general rule, if the circuits of different supplies are likely to be intermingled or connected (such as via signal cables on equipment), then the main earthing terminals should be interconnected. This maintains the equipotential zone and mitigates shock risks between systems as well as reducing signal interference or other EMI issues within the temporary system. See also BS 7671 Regulation 411.3.1.1 (second paragraph).

How do you manage the risks of someone stepping into an IN-S system environment from outside it?

There is no risk.

What would be an example of an IN-S installation - something like a generator powering a 1 day festival stage and there isn't anywhere obvious to connect the Earth of the generator to?

An example often used is location filming where the production will be in use for a few hours, with the generator typically parked in a street. The risks of deploying an earth electrode in unknown ground has to be considered, and if used, it will need measuring. So for a short duration, an IN-S arrangement with the additional RCD in the supply will help protect from cable faults. Note that in the Practitioner's Guide to Temporary Power Systems, a 'short duration' is undefined.

What other measures can be taken where a temporary generator is connected to run industrial plant, forming a TN-S system, however there is a presence of high earth leakage current within the existing plant causing nuisance tripping? As removal of earth leakage protection would be potentially unsafe.

If the RCD is tripping due to normal leakage currents, the RCD is of the wrong rating. See BS 7671 Regulations 132.1 (ii) and 531.3.2. Remember that 30mA devices are used for additional protection for electric shock on circuits of 32A or less to protect the user of connected equipment. It may not be appropriate on a distribution circuit.

For a militarised 2kW generator supplying more than one piece of equipment, we understand that in accordance with BS 7430 the generator should be earthed. We are attempting to get the installers to measure the earth resistance as opposed to just knocking a spike in soft ground as we are interpreting the need to measure as mandatory, unless they measure then how would they know they are in accordance with the guidance – firstly is this assessment correct?

Secondly, we also believe that the resistance of the independent earth electrode should not exceed 20Ω IAW BS 7430; however, 1667Ω is stated in BS 7671 i.e. for a 30mA RCD? There is also the note to Table 41.5 in BS 7671 which states that the resistance should be as low as practicable and a value exceeding 200Ω might not be stable. It’s a bit unclear as to what the measured value should be; any guidance on this subject, please?

Regarding the first question, the generator would need a connection with Earth, if Earth is to be used for fault detection. It would be appropriate to consider the application, the protective devices on the generator output and the generator earthing topology. Given the need for rapid deployments in a range of environments, the design process should consider un-Earthed arrangements as a more practical option. Note also that BS 7430 is a Code of Practice and is not mandatory.

The value of the electrode is covered in the presentation. In essence, the 20 Ohm value is somewhat historical and stems from the values required for HV network protection and doesn't apply here. The values quoted in BS 7671 are for the consumer electrodes in TT earthing arrangements and again are not applicable here. The value of the electrode should be low enough to allow a fault of negligible impedance between a line conductor and Earth to allow the protective device on the generator output to operate in the required time.

With the outside broadcast vehicle what happens with external equipment such as microphones, or more importantly equipment inside the 'theatre'?

The mics and other equipment are not generally a problem. It depends if the OB truck is powered from the building (and if so, does it have a transformer providing simple separation), or if powered from an external generator. BS 7909 provides guidance on how to manage this.

I--S looks similar to IT Earthing system. I didn't see any mention of Insulation Monitoring, that you would expect with an IT system.

Indeed I--S is similar to IT, but it is missing the earth electrode, and hence the 'T' in IT. IT systems require a connection with earth, albeit of a high impedance, in order to detect insulation faults to Earth. With temporary systems the reliable deployment of an electrode of known resistance to Earth is problematical. For that reason if the generator topology is I--S, Electrical Separation is the preferred protective measure.

What is the recommended protective earthing method for a temporary power generator set, which have 2 or more alternators in the same chassy like a canopy or ISO container? The two alternators are feeding to the same BUS and are loadsharing. This operation method will generate disturbances in the earth leakage sensing and suffer from nuisance tripping constantly.

Fit the earth leakage sensing circuit after the point of common coupling of the alternators to prevent circulating currents causing unwanted operation. See the Practitioner's Guide to Temporary Power Systems for examples.

Under what circumstances should you use IN-S rather than TN-S?

IN-S is used in situations where it is not possible or practical to deploy an electrode, such as difficult ground conditions, or where a connection with Earth is not necessary, such as a generator on-board a vehicle for example.

What's a practical solution for the "courtyard problem" where, say, a school's PE dept or a local arts centre throw an extension lead out the window to power multiple items? E.g. musical equipment and PA of mixed classes.

The risk here is one of the failure of the neutral in the supply to the building, but only if the supply is TN-C-S (i.e. PME). If that happens, people outside would be at more risk of shock, being outside the equipotential zone of the building. For a short duration of a couple of hours the risk is very low and, following your risk assessment, may be considered tolerable. If it cannot be verified that the supplies to the lights are RCD protected, then inline RCDs (or similar) should be used for additional protection, though note that this will not help in the event of a PEN failure.

In an IN-S system, if there is no reference to earth what stops voltage drift (i.e. increased voltages) occuring on generator output terminals?

The regulation of the alternator output is a closed control loop and unrelated to the mass of Earth.

The connection of a winding to the mass of Earth (or not) will not affect the regulation.

How might you get a good electrical connection to existing metalwork like a fence without drilling a hole in it?

The same (or a similar) way as you might fix one of your lights to a truss…..

As with all connections, clamps must meet the requirements of Regulation 526.1, which states that every connection must provide durable electrical continuity and adequate mechanical strength. Regulation 526.2 meanwhile, requires that the connection be suitable for the conductors. As the designer, you would need to be satisfied that the clamp is suitable for the connection in terms of its shape, csa and dimensions.

Is it acceptable to connect the generator to the MET of an installation?

Yes, but not as the means of earthing for the generator unless it can be guaranteed as reliable and is not a TN-C-S supply. See BS 7909 or IET guidance on Temporary Power.

What is a reasonable resistance of an earth electrode?

See the presentation. The value of the electrode needs to be low enough to operate the protective device on the output of the generator in the event of cable damage giving rise to a line fault of negligible impedance to Earth.

Is there any advice before attending the BS 7909 course?

Be comfortable with ohms law and general electrical principles. It is a faced-paced course so it helps to be familiar with the basics.

What is the maximum cable temperature for a temporary electrical installation? Is it 60 degrees as per H07RNF spec or can it be 70 or 90 if it is fixed and supported?

It depends on the manufacturer of the cable as to the exact requirements. Note that 'Fixed and supported' could mean lying on the ground (i.e. 'supported') in a protected manner where flexing is not likely or protected against (i.e. 'fixed').

Is there a rule of thumb for sizing CSA of bonding?

Yes - half that of the earthing conductor. See Regulation 544 of BS 7671.

You mentioned that the earth electrode need not be located at the generator. If it is some distance away, must it be cabled back to the generator earth terminal, or could it be cabled to the nearest distribution board?

It is not possible to give a definitive answer without seeing the set-up, but it would be acceptable to have the electrode near the point of utilisation, i.e. a nearby distribution board. Note that in this case, the protective conductor in the supply cable to that distribution board is in effect the Earthing Conductor and should be sized accordingly.

Is test and certification to BS 7909 compulsory for a music festival electrical system?

There is no legal requirement to test and document an electrical system. However, the HSE's Guidance on the Electricity at Work Regulations (HSR25) notes that if it is not tested and documented, there is no way of proving it is safe and that protection will work in the event of a fault.

Both BS 7671 and BS 7909 provide model forms for the verification of electrical installations. The BS 7909 version is simpler, taking into account that the components in the system are pre-assembled and tested prior to being connected into a working system. So while not compulsory, it is requirement of the appropriate British Standards, which are referred to in HSR25 as routes to compliance with the appropriate parts of the Electricity at Work Regulations.

Does BS7909 cover 1:1 transformer isolated systems?

Neither BS 7671 or BS 7909 cover this explicitly. It also depends on whether it is an isolating transformer or providing simple separation or if electrical separation as a protective measure is the intended purpose. Both my books on temporary power provide more information.

What are the earthing arrangements requirements for a temporary back up supply generator?

See Part 551 of BS 7671. As a general rule generators require their own means of Earthing. If the supply fails, the means of earthing may also be lost so the generator will need an alternative.

Given BS 7671 711.411.4 states that we shouldn't use PME earthing for outdoor exhibitions, should we be disregarding all DNO earth's in supply pillars etc., and creating our own controllable earthing arrangements? Given we can't guarantee that even while the supply may appear as TN-S at termination it may have been jointed upstream to using PEN conductors?

Such an approach would not be unreasonable. It should take into account the duration of use (and hence likelihood of risk) and also the proximity of the nearest electrode to the pillar. Alternatively an additional electrode may reduce the risk.

Would using Comando sockets for temporary installations require disconnection times for portable appliances rather than fixed equipment i.e. 0.4s not 5secs?

No. The disconnection time is unrelated to the type of connector. The general rules of BS 7671 apply, namely that circuits of 63 A or less should have a disconnection time of 0.4s, and those over a time of 5s.

If I have a system (ISO container containing equipment) which can be powered by both mains and on-board back-up generators, do both BS 7671 and BS 7909 apply?

Probably BS 7671 only, assuming there are no temporary electrical systems coming from the unit.

In slide 28 you state the EE forms part of the fault path. Are you talking about using RCDs as the protective devices, since trying to get the Earth Fault path resistance low enough to operate a standard MCB would be near impossible? With this in mind what csa should the cable from the EE to the generator be?

It does form part of the fault path, but only if the EE is being used for fault detection as noted in the presentation. The Earthing Conductor from the EE to the generator can be sized in accordance with Regulation 543 of BS 7671.

Can you give any insight as to updates to BS 7909 and BS 7430?

Can't comment for BS 7430, other than it needs a revision. BS 7909 is being updated this year.

If you have two synchronised generators would you require two earth rods or just one to combine them both?

One is recommended after the point of common coupling to prevent circulating currents. See the IET Practitioner's Guide to Temporary Power Systems for more information/examples.

If earthing to structural metalwork, would a lindaptor clamp or similar be considered appropriate to be used to connect the earthing conductor to?

As with all connections, clamps must meet the requirements of Regulation 526.1, which states that every connection must provide durable electrical continuity and adequate mechanical strength. Regulation 526.2 meanwhile, requires that the connection be suitable for the conductors. As the designer, you would need to be satisfied that the clamp is suitable for the connection in terms of its shape, csa and dimensions.

Might it be worth discussing (even a drawing showing) why earthing is important when supplying 110V site transformers from a generator?

It depends if the system is TN-S or IN-S. A reduced low voltage system would still work effectively of the earth electrode is poor. It would in fact be safer still if the electrode was dispensed with and IN-S used instead.

With a trailer-mounted generator parked in a tarmac car park for a TV shoot, what's the best bet in terms of ensuring earth?

See the presentation. IN-S may be a suitable solution.

Diversity factors on temporary systems, is there any guidance you can give? Aside from using data loggers?

As a guide, in temporary systems, diversity is not applied. Typically supplies are put in for specific loads and so the systems are designed around a full-load scenario. That said, not all loads will be on at all times so load-on-demand generation is quite popular. With this, synchronised sets are used and more are brought online as demand increases.

Should general hire companies provide better information on the earthing arrangements of the sets they hire out?

The hire company should provide instructions on the topology of the generating set, but it is the responsibility of the designer of the temporary electrical system to decide what to do with it.

Is rubber cable ok for use in prolonged wet or even submerged situations?

Yes, though prolonged submersion is not recommended, but check with the manufacturer of the cable.

How often should a Temporary Power Distribution system be electrically tested?

As often as the designer thinks appropriate. The environmental conditions and skill of the users would be key drivers in assessing the inspection and testing intervals.

If a temporary installation is planned, factory designed and tested, does the installer always need to be an electrical qualified person or just someone capable of carrying out the necessary test? If yes how does this apply to private individuals e.g farmers or market stalls?

It is difficult to give a definitive answer to this question without more information. But in essence, if a temporary system is planned, assembled and tested, then re-erected at a different location, in the same way, then someone with appropriate training could assemble and verify the system. The 'appropriate training' may not necessarily be formal electrical qualifications but could be company training appropriate for the task for example.