ADS

Coby:

If you simply read the 16th Edition you pick up the principle that I explained to you above.


When you read the Definitions section you become aware that there is still a problem with EEBADS, getting caught with a hazardous live conductor in one hand and an earthed conductive part in the other!

The authors were aware that RCD's were to expensive to insist on at the time, but they knew that EEBADS + RCD's was the answer to the problem. That explains why they used the definitions that they did, assuming that you would be smart enough to understand the basic principle described above!

EEBADS, as applied in the 15th edition, wasn't always the answer to the problem, and sometimes made things less safe.  Bonding every random bit of metal in the house to every other bit of metal, and to the supply earth, can cause electrocutions, rather than preventing them.


Imagine a case where you've just picked up an electric kettle, but the kettle is faulty and the case is live.  You then touch the stainless steel kitchen sink.


If the kitchen sink is bonded, there is a clear path through you from the kettle to the sink and on to earth.  You will be electrocuted.  The current will not be enough to trip an MCB or blow a fuse.

Now imagine a modern kitchen where the sink is not bonded.  There may be some leakage current through the sink, but a lot less than before.  You are more likely to survive.

In both cases, an RCD would probably trip and save you, but only if the circuit has one.  15th edition circuits usually didn't.


So the modern thinking is that if it's not an extraneous conductive part, don't bond it.  And stick RCDs on everything.  ADS still applies to class I appliances, which are always earthed through their supply cable.

Simon Barker:ADS still applies to class I appliances, which are always earthed through their supply cable.

So, this is really where I'm coming from in saying this is a discussion about IEC 61140. BS 7671 provides provision (earthing and protective devices) for ADS for 2 reasons:


1. It's needed for Class I appliances


2. There may be parts of the installation that have conductors protected by basic insulation, installed in conductive enclosures or other equivalent wiring systems (SWA or MICC or single-insulated conductors in conductive containment arrangements effectively equivalent to Class I)


BS 7671 does not include provision for ADS to provide 100 % protection against electric shock.

Simon Barker:

Bonding every random bit of metal in the house to every other bit of metal, and to the supply earth, can cause electrocutions, rather than preventing them.


Imagine a case where you've just picked up an electric kettle, but the kettle is faulty and the case is live.  You then touch the stainless steel kitchen sink.


If the kitchen sink is bonded, there is a clear path through you from the kettle to the sink and on to earth.  You will be electrocuted.  The current will not be enough to trip an MCB or blow a fuse.


Now imagine a modern kitchen where the sink is not bonded.  There may be some leakage current through the sink, but a lot less than before.  You are more likely to survive.


In both cases, an RCD would probably trip and save you, but only if the circuit has one.  15th edition circuits usually didn't.


So the modern thinking is that if it's not an extraneous conductive part, don't bond it.  And stick RCDs on everything.  ADS still applies to class I appliances, which are always earthed through their supply cable.

You mean like this?




Whilst I take the point, aren't most steel kitchen sinks bonded by virtue of the pipes to the metal taps? (Not forgetting the supplementary bonding between hot and cold!)


Any road, you'll be OK if you plug in the kettle to the SRCD by the back door. ?

Chris Pearson:

Whilst I take the point, aren't most steel kitchen sinks bonded by virtue of the pipes to the metal taps? (Not forgetting the supplementary bonding between hot and cold!)


Any road, you'll be OK if you plug in the kettle to the SRCD by the back door. ?

Maybe.  But bonding the sink won't make anything any better.  The kitchen fitters who fitted my sink put plastic inserts in the water pipes.  It must be common these days, because the bathroom fitters did the same thing.

Coby:

It is very strange that a group of electrical engineers would get together and decide to use earthed equipotential bonding in order to limit touch voltages, then call it earthed equipotential bonding and automatic disconnection of supply when they saved you all of this confusion by simply calling it earthed equipotential bonding and the limitation of touch voltages!



 

I think with respect, you have missed the point. EEB on its own does what you describe, and could I agree have been called that - for that is all that EEB accomplishes .

However, to omit the ADS part, that is to say not to have any fuses, circuit breakers or RCDs to automatically disconnect the supply if there was a fault, would be very reckless. Did you really intend to recommend that ? certainly no edition of the regs ancient or modern does. The right sort of ADS works without EEB, but EEB without ADS is really not such a clever idea. If you did mean to propose that, then I think you will be in a group of one !

Coby:

It is very strange that a group of electrical engineers would get together and decide to use earthed equipotential bonding in order to limit touch voltages, then call it earthed equipotential bonding and automatic disconnection of supply when they saved you all of this confusion by simply calling it earthed equipotential bonding and the limitation of touch voltages!



 

They don't call it EEBADS any more. It's ADS now - and has been since 17th Edition in 2008.


The error in terminology has been corrected.


Earthed equipotential bonding is part of ADS and is used to limit touch voltages between exposed-conductive-parts and extraneous-conductive-parts. Importantly, there is no guarantee that main protective equipotential bonding will limit touch voltages to a non-hazardous level - that doesn't come in until you use supplementary local equipotential bonding.


Supplementary local equipotential bonding is used to limit touch voltages between exposed-conductive-parts and between exposed-conductive-parts and extraneous-conductive-parts of the location in the event of a fault, either because it's a special measure for additional protection implemented by a Section in Part 7, or as a measure where disconnection times for ADS in accordance with Chapter 41 cannot be met, or where the designer opts to use it for additional protection for some other reason.


 

mapj1:

 If you did mean to propose that, then I think you will be in a group of one !

I am starting to get flashbacks to discussions in the threads in the old Forum fuelled by someone who shared a surname with a popular brand of port?