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Electric Shock and Prevention webinar

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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

When calculating the average resistance of a body why do we not have a figure for the torso? Or is that included in others?

No values are available for the torso alone.

Do DC currents present different risks to AC, such as from electric vehicle batteries?

Yes IEC 60479 has different tables for AC and DC currents.

Is voltage not inversely proportional to current?

No. Current is proportional to voltage. Current increases with voltage.

I work in the entertainment industry, currently, the productions use a mixture of 400A 0.2 BAC & Powerlock flexible cables for their distribution equipment, these are plugged into stage panels. My question is should the stage panel supplies be RCD protected.

BS 7671 requires sockets rated up to 32A to be 30mA RCD protected. No requirement for above 32A. But you may wish to consider it but leakage currents may trip the RCD.

What is the basic protection provided for swimming pools/or water areas?

The requirement for swimming pools is set out in BS 7671 Section 702.

Why aren’t safety pendant lamp fittings standard in product standards?

Probably because millions of them are fitted and there is no requirement in BS 7671 for them to be fitted as standard.

With reference to Obstacles, a box enclosing live conductors with a lid can only be described as a barrier when it can only be opened by the use of a key or tool. Is that correct? If one can open the box that is secured only by hand tight wing nuts easily, it is only an obstacle.

Enclosures and obstacles are different things. If the enclosure is fitted with wing nuts that are not acceptable as there is no need for a key or tool.

What is the current value where and above it is considered to be fatal to the human body flowing through?

It depends on the current and duration as described in the IEC 60479 time-current curves.

Is there any data to show how many lives have been saved by rcds?

We are not aware of any data that is available.

Can you please re-explain the RCD 30ma limit for additional protection and how this comes about?

It depends on the current and duration as described in the IEC 60479 time-current curves.

With regards to rubber mats does that mean we do not need to comply with IEC 61111/ASTM D 17?

The use of rubber mats should come from a risk assessment or prescribed requirements for particular locations.

How do you reconcile electric shock risk in IT earthing installations?

IT installations such as in medical locations are separated from earth or IT systems may be earthed via an earthing impedance. Where not designed to disconnect on a first fault then an IMD or RCM shall be fitted. See BS 7671 411.6.4.

Is there a good guide to aid the selection of the right type of RCD? i.e. type A or type AC or others.

BEAMA publish a very good guide. Available free online.

What is the basis in the safety of the protective earth being disconnecting in relation to protection against electric shock with EV charging - could this lead to a dangerous situation where the bodywork of the car remains live if the earth path is incomplete (Reg: 722.411.4.1)?

This is done when a loss of Pen is detected. The earth and live conductors are all disconnected together.

Is it correct that currents above 30 mA actually damage cells in the flesh?

Both the magnitude of the current and the duration must be considered.

I was always taught that to avoid the impact of (or reduce the likelihood of) electric shock you should ensure that there is 1m clear space in front of DB's. This was reputed to have come from a "metrification" of the 3ft requirement in the factories act.

That legislation was repealed. Regulation 15 of the EAWR 1989 only requires adequate space.BS 7671 Section 729 provides distances for Maintenance Gangways.

Would you recommend earth pipe bonding as well as an RCD at home or if you have an RCD does this mean you do not require additional bonding?

Metallic pipes entering a building that could introduce a potential must be bonded to earth.

Could we go back to having Regs book available as loose leaf version as it used to be to help save the planet and the SHOCK of cost of the book?

You might want to consider using the IET Online Vital Source instead of a paper book.

Are any diagrams available that show the increase in the risk of severe shock as the frequency increases?

IEC 60479-2 describes the effects of current up to 10kHz.

What is your opinion on the requirements imposed under BS EN 50110?

This standard complements BS 7671 and relates to working on or near electrical installations.

Can high current in SELV circuit still be dangerous?

Yes shorting high current supplies can cause arc flash burns.

Has consideration been given to installing isolation transformers in households? Is there an indication of how many people suffer from electric shock in household/office/workshop environments?

An installation properly designed and constructed to BS 7671 will protect people in homes and workshops.

When I was lecturing, none of the apprentices on house builds were given a design they always use ring 2.5 lights 1.5 cooker 6mm and shower with no adjustments for thermal constraints so most fail EICR. They seem to use On Site Guide.

We are surprised that the apprentices were not taught to carry out simple cable calculations!

Is the installation of an RCD / RCBO (30mA or less) mandatory on a lighting circuit for protection against electric shock?

Yes in domestic premises and in bathrooms and other special locations.

For IT earthing systems is the second earth fault risk for two earth faults on the same phase or different phases?

A simultaneous fault to earth would result in operation of one or more over current devices.

Is it permissible for skilled and competent persons to work on live electrical systems, with exposed conductors, i.e. switchgear and panels?

Regulation 14 of the EAWR forbids working on or near live parts unless it is unreasonable in all the circumstances.

If this is absolutely necessary then Risk assessment and Method Statement is required. PPE.

When plastic water incomer goes to metal and fastened to wall internally that is in contact to to solid floor what is the difference?

The difference is that it is inside the premises. An extraneous conductive part is not part of the electrical installation, conductive, goes from inside to outside and touches the ground or goes to another building so could introduce a potential including earth potential. If the pipe is isolated from outside but then goes back into the floor slab, say below any DPC, then it could be an extraneous conductive part.

For your interest as John mentioned earlier, there is no requirement for rubber mats in BS 7671, however, there is a requirement in the SOLAS regulations (Safety of Life at Sea) to have rubber matting at the front and rear of switchboards.

Thank you for that information. Very sensible given the increased risks.

Have the supplementary bonding requirements for bathrooms been taken out of the BS7671 requirements where 30 milliamp RCDs are fitted?

No. There is still a requirement for supplementary bonding in bathrooms in BS 7671. However, it may be omitted if certain conditions are met. See BS 7671 701.415.2.

Do we need to bond metal door/window frames?

Generally no, as they are not extraneous conductive parts. But yes for windows and doors in Transportable Units see BS 7671 Section 717.

I was always told that in order to reduce the effects of electric shock you should ensure that there was 1m clear space in front of a DB, this was reputed to have originally come from a 3ft requirement in the factories act. I cannot find this written directly in BS7671 nor can I find it in the MoD document (now CIBSE DG08). It is hinted at under "Gangways", but where has this requirement gone to? How can it be referred to?

Section 729 of BS 7671 describes dimensions for maintenance gangways.

If you have a high ELI on the incoming supply on a TNS system say 0.89 ohms 100 amp BS 1361 type 1 is it necessary for an additional 100mA RCD if you are using a CCU with duel or all final circuits protected with additional 30 mA protection which is in compliance with 18th.

If the Ze is too high, with any required bonding connected, for the cutout fuse to meet a 5s disconnection time then treat as a TT system. Insulated and sleeved glands enter the consumer unit through a plastic bush. Leave sheathing on and go direct to the main switch. If all RCBOs are not used then any internal wiring of the consumer unit to internal RCDs need RCD protection in the form of a BS EN 61008 S Type main switch.

Live working guidance in HSG85 from HSE.

Thank you we agree.

If an enclosure has been isolated and a Permit to work issued by and AP but there are parts that can be touched which are live, what precautions should be put in place and should the permit detail this?

Regulation 14 of the EAWR forbids working on or near live parts unless it is unreasonable in all the circumstances. If this is absolutely necessary then a Risk assessment and Method Statement is required. PPE.

If I recall correctly, the last time I checked the BC & ES product standards they now require that all new lamp holders are safety lamp holders?

Only lamp holders meeting the requirements of BS 7895.

What always worries me is a lamp holder on a table lamp.

Yes hopefully plugged into a 30mA RCD protected socket.

Don't the bayonet holder live contact have spring connections so when there is no bulb fitted they are not live?

No. Only lamp holders that have enhanced safety, meeting the requirements of BS 7895.

Re meter box - this is not an Enclosure (Pt 2) but more of a "Compartment" which is mentioned in Part 5.

We agree it is not an enclosure for basic protection.

With regards to SELV, does not exceeding 120v DC as a form of protection mean that we do not need to meet max ZS for protective devices? ie a 110v DC central battery emergency lighting system.

SELV is separated extra-low voltage. Separation means separated from earth so you cannot measure Zs.

Can you please review the question concerning CLass1 and 2 appliances?

I think the question was the one relating to earthing Class 1 appliances and Class 2. Class 1 equipment is an exposed conductive part so requires earthing. Class 2 is a method of protection that does not require earthing.

Surely Class 1 equipment has a connection to earth?

We agree. Exposed conductive parts require a connection to earth.

The shock the person got from angle grinder was almost certainly static electricity build-up.

Possibly but we advised withdrawal from service and an inspection and test just in case the grinder was faulty.

Just a point that metallic pipework may use plastic fast fit connections. How do we deal with this scenario is it go back to EEBADS (Earthed Equipotential Bonding and Automatic Disconnection of Supply)?

A continuity test across the joint to verify if the pipe is an extraneous part is advised. More than 22kΩ will mean the pipe downstream does not require bonding.