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In wet locations a body resistance of 200 ohms is assumed, and disconnection times go down to 0.2 seconds in a 230 volts supply (0.4 110 volt supply), is that correct? Or is this not required in BS7671?
 

loz2754

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No it's not.
The disconnection time for TN systems is stated as 0.4s, regardless of whether it's a wet location or not.
Or for TT systems, 0.2s.

Additional protection is required, by supplementary bonding and/or RCD rated at 30mA.

The wet locations as laid out in BS7671 would be either rooms containing a bath or shower, or swimming pools, or saunas. Each of these locations has its own chapter dealing with specifics for those locations.
 

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No it's not.
The disconnection time for TN systems is stated as 0.4s, regardless of whether it's a wet location or not.
Or for TT systems, 0.2s.

Additional protection is required, by supplementary bonding and/or RCD rated at 30mA.

The wet locations as laid out in BS7671 would be either rooms containing a bath or shower, or swimming pools, or saunas. Each of these locations has its own chapter dealing with specifics for those locations.


I'm a bit surprised by this. IEC61200-413 states that in wet locations the body's resistance is assumed to be 200 ohms with a touch voltage limit of 25 volts.

I would imagine this would apply to unfinished basements, garages, outdoor, kitchens, laundry rooms and bathrooms where a person may be barefoot, wet, ect having a significantly lower body resistance.

How does BS7671 deal with these individual locations?

Supplementary bonding merely extends the voltage to a larger area, (and in some cases remote earth or an unbonded object may still poke through) though in theory as long as it does not exceed 0.4 seconds outside the wet area it does/can make sense. Regarding RCDs they are simply a back-up and should not be used as primary protection in that they can and do fail.
 

loz2754

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Interesting questions raised.

In the case of a bathroom, which is classed as a special location, the purpose of the supplementary bonding is to ensure that any simultaneously accessible exposed or extraneous conductive parts are at the same potential, thus minimising shock risk to a person with damp skin who happens to touch more than one such item at the same time.

The other locations you mention, basements, kitchens etc are not specifically classed as wet locations. In which case, the general rules of BS7671 apply, ie automatic disconnection, selection and erection of equipment according to the nature of external influences, etc.

Construction sites have their own rules, either limiting voltage to reduced low voltage (55V to earth), and or the use of RCDs.

I'm sure all the bases are covered one way or another
 

timhoward

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BTW The older 16th edition guide to special locations is summarised online here:
if the OP wants to browse to get the general idea.
 

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Interesting questions raised.

In the case of a bathroom, which is classed as a special location, the purpose of the supplementary bonding is to ensure that any simultaneously accessible exposed or extraneous conductive parts are at the same potential, thus minimising shock risk to a person with damp skin who happens to touch more than one such item at the same time.

The other locations you mention, basements, kitchens etc are not specifically classed as wet locations. In which case, the general rules of BS7671 apply, ie automatic disconnection, selection and erection of equipment according to the nature of external influences, etc.

Construction sites have their own rules, either limiting voltage to reduced low voltage (55V to earth), and or the use of RCDs.

I'm sure all the bases are covered one way or another


Any idea why they would not be classed? Is bonding typical in UK bathrooms? Particularly the floor rebar?

A person does not have to touch two objects at the same time (I assume you mean hand to hand here) as they are standing on the floor. Hand to foot is about 200 ohms. At 115 volts this comes to 575 milli-amps. A 0.4 second disconnection time puts a person in that AC 4.2 section of the IEC 60479-1 body graph- 50% probability of ventricular fibrillation.

Under the NEC these areas are required to have GFCI protection in that back in the day ungrounded appliances were documented as resulting in electrocutions at 120 volts.
 

Julie.

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Any idea why they would not be classed? Is bonding typical in UK bathrooms? Particularly the floor rebar?

A person does not have to touch two objects at the same time (I assume you mean hand to hand here) as they are standing on the floor. Hand to foot is about 200 ohms. At 115 volts this comes to 575 milli-amps. A 0.4 second disconnection time puts a person in that AC 4.2 section of the IEC 60479-1 body graph- 50% probability of ventricular fibrillation.

Under the NEC these areas are required to have GFCI protection in that back in the day ungrounded appliances were documented as resulting in electrocutions at 120 volts.
But it's not 0.4s disconnection time.

Locations containing a bath or shower (or pool) must have rcd protection, that's usually <40ms disconnection time.

There is no concept of "wet areas" or bathrooms/kitchens/utility rooms in bs7671 - it's whether the Location contains a bath or shower, outside of this bath/shower zone it's just like any other room.

In reality in every residence pretty much all circuits end up having rcd protection, so every area has <40ms disconnection time for earth faults
 

Lucien Nunes

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Hand to foot is about 200 ohms. At 115 volts this comes to 575 milli-amps. A 0.4 second disconnection time puts a person in that AC 4.2 section of the IEC 60479-1 body graph- 50% probability of ventricular fibrillation.

It's worthy of comment that when risk assessing this situation in a bathroom, comparing with other countries' regulations, the UK does not and never has permitted normal electrical outlets or switches in a bathroom. The scope for coming into contact with appliances and accessories connected to the TN supply is therefore much reduced and with it the likelihood of shock to earth. Only pull-cord switches with an insulating string, and shaver outlets fed via an isolating transfomer (I think limited to 20W) are permitted. We cannot plug a hairdryer in, period.

In the days before universal use of RCDs for all circuits e.g. under the 16th edition, bonding was widespread. Radiators, taps, pipes, bath, anything metal that could stand even the tiniest chance of being extraneous got bonded.

Likewise as mentioned above, the UK does not have the same concept of 'damp areas' that exist in many countries' regs. We expect a garage to be dry enough to use indoor accessories and methods. If it isn't, everything needs to be full-on outdoor spec.
 

timhoward

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It's worthy of comment that when risk assessing this situation in a bathroom, comparing with other countries' regulations, the UK does not and never has permitted normal electrical outlets or switches in a bathroom.
I think I disagree! My 18th Edition course said normal switches ok outside zones and if you have a palatial bathroom and can achieve 3m outside zone 1 a socket is ok too.
I'm going to have to go and check now....
 

Lucien Nunes

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Yes indeed, sloppy posting on my part. I should have said permitted in bathroom zones now and previously did not permit at all. In reality you will not see a socket or rocker switch in a normal bathroom.
 

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But it's not 0.4s disconnection time.

Locations containing a bath or shower (or pool) must have rcd protection, that's usually <40ms disconnection time.

There is no concept of "wet areas" or bathrooms/kitchens/utility rooms in bs7671 - it's whether the Location contains a bath or shower, outside of this bath/shower zone it's just like any other room.

In reality in every residence pretty much all circuits end up having rcd protection, so every area has <40ms disconnection time for earth faults


I guess that is what they are replying on, but me personally, I wouldn't in that RCDs can and do fail.
 

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Yes indeed, sloppy posting on my part. I should have said permitted in bathroom zones now and previously did not permit at all. In reality you will not see a socket or rocker switch in a normal bathroom.

No sockets in UK bathrooms? Why not? And why not rocker switches?
 
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