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UPS Systems and Generators webinar questions and answers

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This webinar was arranged by the IET’s Electrician EngTech programme in association with Wiring Matters. Electrician EngTech helps electricians gain recognition of their professional competence.

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Your questions answered.

What earthing strategy should be used when transitioning between mains and generator (period UPS runs on battery)?

It depends on the design strategy adopted and end client's break or no-break requirements. Single point earthing principle based on relevant parts/sections of BS 7671 and BS 7430 should be complied with for transitioning between mains and generator to maintain neutral-earth reference.

You have described uninterruptable power supplies, but there are also 'offline', 'online' and 'hybrid' types?

There are both online and offline but generally online is used for IT and sensitive equipment support as it has effectively no time delay in switching over, but offline can have a short delay which may not suit all equipment. An offline UPS may be less expensive but the user and installation designer will need to consider the operational requirements.

What is VRLA?

Valve Regulated Lead Acid (battery).

Do battery sets have to have a hazardous zone (DSEAR) around them?

If batteries release flammable gasses when charging, this needs to be safely vented to atmosphere to prevent danger of explosion. If an assessment shows that a hazard could be present under abnormal operating conditions (e.g. loss of vent fan) it would be Zone 2.

How should the surge withstand capability of a UPS be specified?

The UPS will provide some surge protection to the load but for proper protection the installation UPS and its downstream equipment should meet the requirements of BS 7671 Reg 443.6. Surge protection for the UPS and its downstream equipment shall comply with Section 443 of BS 7671 and relevant requirements of BS EN 62305 Parts 1-4

Similar to N+1 Gensets, Please explain N+1 UPS.

Regardless of generators or UPS, if it is required to have N number of units to meet the demand, for maintenance or breakdown contingency, 1 additional unit has to be provided to allow 1 of the N units being taken out of services under either fault or maintenance conditions.

4 pole or 3 pole and linked neutral .. to avoid dangerous voltages arising. Isolator for the 'island' mode; and the effect on downstream UPS system.. Just wanted to have your opinion on this please.

For equipment downstream of UPS, it is necessary to risk assess any danger of circulation of neutral current in using auto-change-over switching from mains to UPS feed; designer/installer will use this information to decide to use 4-pole or 3-pole-linked neutral as appropriate.

At the mains supply, single point earthing principle based on relevant parts/sections of BS 7671 and BS 7430 should be complied with for transitioning between Mains and Generator to maintain neutral-earth connection reference.

Break before Make or Make before Make?

For standby applications, ENA G99 relay operation should seek approval from your local DNO. This will govern the necessary break before make requirements of transition between mains to genset and vice versa.

Please explain the application of standby genset being used for feeding back into the grid, and what governs this application of the plant?

Under normal operation conditions - i.e., when the mains is offline, the genset is only for standby purpose, it can never be configured to feed back to the grid.

Genset for Data Centre application should be rated for continuous operation, is it not? (as per definition used in Uptime Institute)

In the US, their electricity distribution network (mostly are overhead lines) is far less reliable in uptime and quality than the one we have in the UK.

The UTI requirement is quite rightly treating the US DNO supply as a secondary standby source, hence it is asking for the generator units to be the primary power source (not for standby purpose); and it needs to have the ability to operate 8,760 hours a year and for the foreseeable lifetime of the data centre.

In GB, the UK National Grid is a lot more reliable and normally can be considered as the primary supply source for the data centre; the gensets would be simply a secondary standby source only expected to be running much less than 8,760 hours a year. 

It will be up to the end client and designer of the Data Centres in different countries to agree with the appropriate UTI certifying authority to define whether the generators shall be assigned as the secondary standby or primary power source. 

Which type of UPS is justifiable for medical equipment (local UPS or centralised UPS)?

In addition to compliance with HTM and BS 7671 Section 710 requirements, designers/installers should undertake appropriate risk assessments to determine whether local UPS or a centralised UPS solution will eliminate life safety threats.

Use local UPS's for small local equipment in areas such as theatre as one large UPS would create problems if it failed.

Is it a legal requirement for a lift on an industrial site to have a backup power system?

If the lift has been identified and being assigned as life safety equipment under fire regulations/standards, then it is required to have a backup power supply.

How can you carry out maintenance on distribution systems which are deemed critical and have multiple sources of power such as DNO, UPS and Genset as clients are not very keen to shutdown all systems.

In many HSE guidance document on electrical systems, it is required to ensure equipment including switchgears and cables of HV and LV distribution systems be provided with isolation and earthing down facilities for routine and fault maintenance.

Additional safety facilities such as Castell key interlocks might also be deemed necessary to prevent the part of the circuit/system to be energised inadvertently due to multiple sources of power existing in the system. If designers/installers were proved to have not made such provisions and should an injury occur, HSE can prosecute either the company or the individual who failed to provide such safety facility.

Remember ignorance is not an acceptable defence under Electricity at Work Regulations. If an installation cannot be shut down and maintained safely it will eventually fail but that is the installation owner's risk.

What load acceptance % can affect a standby genset to run up to operational synchronised speed?

A crashed synchronisation i.e., the genset will fail to operate as it was pull out of the correct running synchronisation speed - this will occur if the genset is being asked to take on loads in excess of its load capability during start-up.  Ideal load steps for diesel gensets will be 40%, 30% and 30%.

The genset manufacturer should be consulted for a higher load percentage to be switched on after the mains failed and the standby genset is on line.

What is the specific regulation which requires hotel end disabled lifts to have a backup?

BS 9999 Annex G. Such requirements may also be in national building or safety regulations, or in a client's general requirements.

Would you always switch the neutral on an ATS when a genset is a backup to a mains supply?

It depends on the design strategy and end client's break or no-break requirements. Single point earthing principle based on relevant parts/sections of BS 7671 and BS 7430 should be complied with for transitioning between mains and generator to maintain neutral-earth reference.

In paralleling of multiple gensets, what can be done to avoid circulating currents in the grounded neutrals?

There are a number of proven arrangements to avoid circulating currents in the grounded neutrals.

Please consult BS 7430 and IEEE Green Book (i.e., IEEE Standard 142) Recommended Practice for Grounding of Industrial and Commercial Power Systems for details of those arrangements.

Regarding auto-transfer switch: can auto-changeover contactors be used as an alternative for gensets?

Of course it is possible to use auto changeover contactors as well as ATS for gensets. However, the utilisation duty (AC1 to AC4) of the electrical contractors is categorised by the types of electrical loads used such as inductive, or resistive or capacitive and duty cycle such as plugging or runtime breaking, short circuit breaking etc.

If contactors were to be used, it will need to be specified correctly the utilisation category to longevity operation use.

For UPS systems we can use lead acid or alkaline battery cells. Both types have specific ventilation requirements?

Any type of secondary batteries can be used as long as the UPS is designed for their use. Generally lead acid types are used as they are relatively inexpensive, easily obtainable and can be disposed of and recycled relatively easily.

How do you typically comply with disconnection times for a generator? Earth fault relay?

Most generator controllers will have overcurrent settings which can be adjusted to coordinate with generator thermal damage curves in order to limit the amount of overcurrent that can occur.

The controller overcurrent protections can be set up to work in coordination with downstream protective devices in the system. For downstream device disconnection, BS 7671 overcurrents and fault protection requirements have to be satisfied by discrimination study.

For generator internal faults such as stator winding to earth fault, an earth fault relay with CT at the neutral would be necessary.

Do we need to alter the Gen set ACB settings when carrying the discrimination study?

Normally a genset manufacturer will provide a genset MCCB or ACB and these will be set at their default settings. Designers/installers are to conduct a discrimination study to confirm or alter the default setting to coordinate with downstream devices to meet BS 7671 overcurrents and fault protection requirements.

Do UPS installations require a dedicated independent earth nest?

No, it is not permitted - as this UPS earth nest can be at a different potential to the general installation earth. This creates a certain touch potential among equipment in the same building.

The UPS earth nest must be interconnected to the main earth to eliminate such unknown touch potential within the installation.

In the single point earthing config, 4-pole breakers cannot be used?

It depends on the design strategy and end client's break or no-break requirements. Single point earthing principle based on relevant parts/sections of BS 7671 and BS 7430 should be complied with for transitioning between mains and generator to maintain neutral-earth reference.

Where does the 10 ohms value come from? Do all the customer protection settings have to be changed given the impedance is normally below 1 Ohm?

The 10 Ohms value is a maximum value we have inherited from the earliest version of BS 7430.

The discrimination study outcomes should be able to identify whether the protection settings are affected by transitioning from mains to genset or not; it also depends on whether the earthing system is TT or TN type.

For diesel generators, in your opinion, should fuel polishing be specified as standard, to prevent start up issues, excess maintenance requirements, etc. through its design life?

Over time, stored fuel for standby gensets degrade and become infected with water from condensation. This leads to bacteria (fuel bug) developing, thus resulting in higher fuel consumption, damage to engine parts and potentially engine failure.

It is possible to either install a permanent system or outsource it to de-risk the issue. The choice is up to the designer/installer and client.

Will ride-through depend on any motors or motor starters connected?

Fault Ride-through duration is dependent on the type of generators as stated in the GB Grid Code which can be downloaded at

Designers and installers can ask the local DNO to advise them if their genset(s) needs to comply with fault ride-through duration or not.

When a large 3 phase emergency generator is connected to a fixed wiring installation in island mode, can the generator earth be connected using the earth of the fixed wiring installation without driving an earth rod into the ground to obtain a generator earth.

If the fixed wiring installation is in an island mode, the installation will (and should) have its own earthing facility independent of the DNO's supply. As long as the fixed wiring installation has an earth independent of other power source (e.g., DNO), the genset will be permitted by BS 7671 to use it for its earthing purpose.

Consider an installation with DNO supply and a standby genset using 4-pole breakers (separately derived power systems), with the mission critical loads supplied via a UPS. During the momentary transfer of power systems from DNO to standby genset (and vice-versa), the neutral connection to the UPS will be in floating condition. Can please clarify if the UPS may trip due to the floated neutral condition?

As most of the loads that are downstream of a UPS tend to be single-phase loads, the loss of neutral-earth reference of these loads during transition of mains to gensets or vice versa are not recommended.

There were some beliefs that if the three single group of loads are fairly balanced then the short transitional duration will be tolerable by the UPS. However, most UPS manufacturers recommend the use of three-pole switching, or isolation transformers or neutral earth contactors to maintain neutral-earth reference at all times.

Again, it depends on the design strategy adopted and end client's break or no-break requirements.

On modern UPS systems, should capacitors/filters etc be considered consumables in the same way as batteries?

Yes, they will require replacement as a part of maintenance - the UPS manufacturer will be able to advise.

When running the generator in parallel with the mains, can the PFC/harmonic filter remain connected?

Yes, PFC and harmonic filters can remain connected to the system when the DNO mains supply is online (i.e., in parallel with the generator). This is because all the reactive kVA and harmonics will be catered for by the DNO mains supply.

How do you connect the multiple generators to a bus bar?

Typically, each generator will be connected to the same busbar within a multi-section panel with individual ACB or MCCB. The closing and opening of these ACBs or MCCBs will be governed by a genset synchronising control panel linking to a G99 relay panel.

Can you explain in more detail why we should not disconnect the neutral conductor in UPS in a TN earthing arrangement and also what is the case in TT earthing systems?

Firstly there is no difference in a bad outcome can exist in 3 phase 4 wires TN or TT systems when the neutral connection to earth is lost.

Secondly, this is not only in UPS but also for DNO or generator supply as well.

To help you visualise the danger of losing the neutral to earth connection, let's consider drawing a solidly earthed single-phase source supply connecting a single-phase load with the line and neutral conductors connecting to the load impedance within a metallic class 1 enclosure. If you cut out a section of the neutral conductor, you will see that the line conductor will remain energised but the equipment appears to be in an OFF state. This is a danger to the maintenance staff as the neutral terminal of the equipment which is normally at earth potential now is at a 230V potential to earth.

Next, if you draw a solidly earthed three-phase source supply connecting three single-phase loads (one with 1-Ohm, one with 10-Ohms and one with 100-Ohms) with the line and neutral conductors connecting to the load impedances within its own metallic class 1 enclosure, with the neutral to earth connection in place at the source, and connect the neutral conductors of each single-phase loads to a common neutral busbar in a local DB remote from the three-phase source, the current in the three phases are 230A, 23A and 2.3A, the voltage in each phase remains constant at 230V.

Now, if you cut off a section of the common neutral conductor between the three-phase source and the three single-phase loads, the current and voltage for the L1 load become 37.8A and 37.8V respectively, the current and voltage for the L2 load become 36.1A and 361V respectively, the current and voltage for the L3 load become 3.78A and 378V respectively.

Losing the neutral-earth connection with unbalanced single-phase loads can create over-voltage on loads.

What are the differences between selecting a genset for prime or continuous use in terms of operating hours and would it differ based on the application?

Continuous Power is 8,760 hours per year less service downtime, applicable for supplying utility power at a constant 100% load for an unlimited number of hours per year. No overload capacity.

Prime Rated Power is Unlimited number of hours per year (8760 hour/year less service downtime ) for variable load, and not to exceed 70% average of the prime rating, during any operating period of 24 hours.

It can have 10% overload capability in accordance with ISO 3046, AS 2789, DIN 6271 and BS 5514. (ref: Power topic #6240 | Technical information from Cummins, Inc.)

Can a static UPS be used as a life safety supply to a fire fighting lift?

Generally, the use of a UPS is not a good idea as they only have limited autonomy, and one has no idea what may be required in an emergency.

Usually, a generator is provided as they can run as long as there is fuel.

For example, some European hospitals do allow a UPS as a back-up supply for minor operations with a well understood duration, but in the UK a generator is always required.

Could you expand on the use of flywheel based systems please?

Under normal conditions, an electric motor drives a generator that supplies the required load.

On a common shaft the motor also drives a flywheel which stores energy by rotating, and a diesel engine is also connected to the shaft by a clutch mechanism that allows the shaft to rotate without turning over the engine.

At loss of electrical power the flywheel continues to rotate and its stored energy continues to drive the generator for a short while to maintain the electrical supply.

The diesel engine then starts and the clutch closes so the engine directly drives the generator.

Can floating neutrals cause issues on PFC equipment and UPS equipment if the neutral earth is incorrectly set up on changeover to generator power?

Yes, losing the neutral-earth (floating neutral) connection with unbalanced single-phase loads can create over-voltage and under-voltage issues on loads.

What considerations should be given for heat generated by a UPS system? Is there a standard heat figure given per kW of UPS power?

Unfortunately, there are no standard figures and each manufacturer has technology which they use, but the passage of any electric current through a conductor will generate heat!

Location of the UPS for adequate ventilation for cooling is obviously important, and forced ventilation may allow increased power output but then continued ventilation must be ensured.

BS 7671 both allows and disallows UPS bypass - can you clarify the situations this applies please?

Chapter 51 of BS 7671 allows a UPS to operate with the static bypass to clear a fault when a UPS is supplying general equipment such as IT etc, but when the UPS has to supply any life safety system the inverter itself must be able to clear a fault or operate necessary safety devices directly from the inverter supply.

We normally have the neutral taken to the TX (UKPN engineer).

Yes, this is one of the correct connection arrangements. When dealing with a package substation, it is normally permitted to have the neutral to earth connection to be installed at the main LV supply panel.

Will it trip >300ms?

Possibly, this will depend on the protection settings of the generator controller and ACB/MCCB.

Is there ever a situation what there would be a requirement for a neutral/earth transformer on an LV generator installation or is this only a requirement with MV installations?

Neutral/earth transformer is commonly used in HV systems to limit earth fault current and the output voltage to a very low value. This is because a large and long time rated NER will be more expensive.

In LV, it is possible to use a neutral/earth transformer see Figure 11 in BS 7430.

Please highlight use of 4-pole CBs (separately derived power systems).

Please see Figure 10 in BS 7430.

Can most UPS systems handle a loss of the earth/neutral connection?

The acceptance of a momentary loss of earth/neutral connection will depend on the design strategy and end client's break or no-break requirements.

The HK Govt practice was to size the genset rating at least 3 times that of all downstream UPS added together. Is it alright or too much?

This appears to be a specific requirement of a particular client (i.e., HK Government). The sizing criteria of a genset comprising many different considerations, e.g., total essential loads, load steps and switching on time, heavy motors, UPS loads and UPS charging, fault capacity requirements etc. the 3 times of all downstream UPS added together will be just one of the many criteria.

Is it a bad idea to have 4 pole circuit protection upstream of a UPS? Only use 3 pole protection upstream to avoid the neutral floating.

Absolutely agree. It also depends on the adopted design strategy and end client's break or no-break requirements.

Although we are not considering PV at this time, in relation to a standby emergency generator, what is the protocol to prevent the generator being back-fed from the PV?

When the DNO mains failed, under G83 or G99, the PV will be offline. It depends on the design strategy and end client's requirements as to whether the PV is allowed to be brought back on line during lost of mains emergency operation.

Risk assessments should be conducted to ensure there is no detrimental influences on the genset from the PV. I.e., the PV shall not cause the standby genset to trip out.

In terms of recommendation for a protective (MCCB) device for UPS system, what would be the best type of protective device i.e. 4-pole or 3-pole device to be installed?  I am just thinking that a UPS needs a reference point and therefore, the earth would be the reference point.  If this is isolated during isolation of the protective device, this would cause issues.

It depends on the design strategy and end client's break or no-break requirements.

What about earthing for a TT system? What would be the value for RCD based Genset 300mA etc.?

In BS 7671, for protection against electric shock for a person RCD should be 30mA 40ms, for equipment earth fault disconnection can be 100mA 100ms. 

Is 10 Ohms value to do with limiting the EPR under fault conditions?

No, based on the average soil resistivity in England and Wales, the 10 Ohms earth nest resistance is more likely to be able to be achieved in most parts of the UK mainland.

With TT system, and 10 Ohms electrode resistance, will the breaker trip under earth fault condition?

BS 7671 required circuits to have RCD protection when the earthing system type is TT.

I noticed for the testing of the standby generators a load bank was included.

Yes, a suitable load bank (reactive or resistive) either temporary or permanent shall be made available for generator testing use. And in some cases, for UPS testing use depending on the client's requirements.

My experience is that the construction of UPS is more suited to climate-controlled rooms in data centres and offices than industrial situations. Given that VSD’s are generally suitable for industrial applications and the principles of operation of a UPS and a VSD are essentially the same, is there a reason why UPS cannot be constructed in a similar robust fashion to a VSD?

As with everything it depends on the cost. A small UPS could be accommodated in a robust enclosure and installed in a plant area as is a variable speed drive, but if you were wanting to support a large load - say 100kVA - the enclosure and ventilation would be impractical and effectively it would be in its own room.

Smaller modular UPSs are being produced that can be more robust, and manufacturers would always look at such provisions if they see a market. 

Can a neutral earth reference point be provided by a 4 pole breaker from a standby generator with the neutral earth link before the 4 pole breaker i.e. not on the load side of the breaker?

It depends on the design strategy and end client's break or no-break requirements. Please consult BS 7430.

Why does BS 8519 (2020) now recommend the use of UPS as a secondary source of supply to buildings? As they have removed the option of obtaining a secondary supply from the DNO which usually is the option we would go for. Generators (the other option) now require SCRs to meet emission levels (cost around £100k %2B). UPS really struggle with the regen effect caused by lifts (VT).... which are deemed life safety.

Generally it is considered that a second "alternate" DNO supply would come from the same distribution system as the original supply, and so in a significant power outage would be as likely to fail as the original supply.

Which type of UPS is better to use in industry, VRLA or lead operated?

VRLA is widely used as they are relatively inexpensive, give good service, and can be replaced and recycled relatively easily.

Other technologies can be used but the UPS must be constructed to accommodate the battery type used.

What is the amount of h2 emission from UPS? How dangerous is that? What are the precautions to be taken?

The amount of hydrogen liberated in battery charging depends really on the make of battery and its charging rate and these figures should be obtained from the manufacturer.

For safety application, will generators or UPS systems be able to comply with IEC 61508?

According to BS 7671, UPS and standby genset are acceptable to be a safety energy source. Under HTM 06-01, where automatic transfer systems are used, they should be assessed for their safety integrity level in accordance with BS EN 61508 Parts 1–3.

Apart from harmonics can the UPS cause interharmonics and as a result cause Sub Synchronous Resonance on the generator? If yes how could you deal with interharmonics?

Any power electronics equipment can possibly produce harmonics and interharmonics, and depending on the circuit and equipment operation resonance may occur.

Harmonic filters may be applicable, but such applications can only be considered on a case by case basis with the equipment manufacturers.

I am undertaking design and installation of UPS and Gensets within Data Centres - please could you recommend any specific training that I can sign up for?

Several UPS and generator manufacturers run training courses and provide technical literature, but generally this covers their own products only so it may not give an overall view of all installation requirements or practices.

To clarify, I presume this is for regular testing - what of environmental issues?

For standby gensets noise considerations - IEMA Guidelines for Environmental Noise Impact Assessment 2014 and BS 4142 can be used, for environmental issues such as air quality and NOx emissions - can be used.

For testing procedures of gensets - this link may be of some use for reference.

With regard to the generator load banking, if you have G99 can you parallel with the DNO to load test the set?

Load test for the genset would normally be done independent of DNO's supply. i.e., load test for the genset should not be connected in parallel with the DNO's supply.

Does calculating the arc flash current from the output of a UPS differ from that of 3 phase 11kv/415 v transformer?

The principle and equations that are used in calculating arc flash energy are the same.

What is the largest capacity of UPS practically?

The factors that govern the largest rating will depend of the autonomy the UPS is required to run.

Also for larger sizes rotary UPS units may be preferred to static as there will not be any batteries to maintain or replace.

Is a 4 pole up stream of a UPS a definite no-no as the neutral will be floating and cause surge suppression to blow?

It depends on the design strategy and end client's break or no-break requirements. Please consult BS 7430.

Can you advise on when UPS wrap-arounds are a good idea, or not?

Wrap-around is a type of maintenance bypass switch. The provision of a maintenance bypass facility is necessary to enable a UPS system to be electrically isolated and taken out of the critical power circuit for safe UPS maintenance or unit replacement without any disruption to the load. 

Without an external bypass switch to allow preventive maintenance the only safe alternative would be to power down the entire critical load, not just the UPS.

Are we starting to see new types of batteries being developed for UPS?

New and more efficient batteries with higher energy density and smaller volume continue to be developed, mainly for the emerging electric vehicle market but they may be suitable for other uses too.

Any type of secondary batteries can be used for a UPS as long as the UPS is designed for their use.

If you have, say, three generators connected in parallel, and the neutral is resistance grounded, do you need a neutral grounding resistor on each generator neutral?

No, if suitable switching arrangements can be made, you do not require NER for each genset.

It depends on the design strategy and whether  the client ask for N+1 NERs to be provided. Even if N+1 NER is installed normally just one NER is needed to connect to one genset only.

How much to oversize a generator to allow for a UPS? Why is this oversizing required?

There is no one simple rule in sizing (or oversizing) a generator, The sizing criteria of a genset comprises many different considerations, e.g., total essential loads, load steps and switching on time, heavy motors, UPS loads and UPS charging, fault capacity requirements etc.

When running on standby it is necessary for the generator to support the installation and recharge the UPS.

Do you think Lithium-Ion batteries will become more prominent due to their obvious advantage?

Battery technology is continually developing and the limits are generally cost and safety. Technology can already accommodate the use of such batteries in some equipment and as long as the costs are realistic and the installation can be operated and maintained safely there should be no problems.

Fire is always a consideration with Lithium and this must be considered as a part of any fire safety strategy or assessment.  

What considerations should you think about when using the new Lithium Ion Batteries for UPS systems in terms of fire? And what type of fire suppression method should be used? There are many articles coming out at the moment about UPS battery fires due to LI-Ion.

Not being a fire safety expert I'm not able to comment, but I am aware that there has been significant research done on fire extinguishing methods for Lithium. This must be considered as a part of the building's fire safety strategy or assessment.