Bathroom bonding with steel stud walls

[Midwest]

I can't understand the point of bonding the metal stud wall construction. Some of the installations I've seen, you would have to run several cables to effectively bond the studwork. The requirement for RCD protection for cables installed in such an installation (with no earthed mechanical protection), came in for the 17th Ed, perhaps because of this incident?

So OP, bonding your metal stud work, seems ineffective and unnecessary. Why have you bonded the metal pipes?
You may have made something that was not extraneous now more dangerous than it was before.

 

[Richard Redman]

I can't understand the point of bonding the metal stud wall construction. Some of the installations I've seen, you would have to run several cables to effectively bond the studwork. The requirement for RCD protection for cables installed in such an installation (with no earthed mechanical protection), came in for the 17th Ed, perhaps because of this incident?

So OP, bonding your metal stud work, seems ineffective and unnecessary. Why have you bonded the metal pipes?
You may have made something that was not extraneous now more dangerous than it was before.

How can it be more dangerous than it was before ?

 

[Deleted member 26818]

Well, by bonding the steel studs, you will now have fault current inside the wall if there is an earth fault in the installation.
As long as the steel studs are mechanically connected, there shouldn't be a problem with sparks jumping from one to the other.

 

[Richard Redman]

Well, by bonding the steel studs, you will now have fault current inside the wall if there is an earth fault in the installation.
As long as the steel studs are mechanically connected, there shouldn't be a problem with sparks jumping from one to the other.

Right ok.

Well, by bonding the steel studs, you will now have fault current inside the wall if there is an earth fault in the installation.
As long as the steel studs are mechanically connected, there shouldn't be a problem with sparks jumping from one to the other.

Ok, so this is what i have done.
New electric shower has been fitted, and a new circuit from 40A breaker RCD protected.
The pipework to the bath is bonded, but some plastic fittings have been used here and there, so have run a 10mm earth cable from MET to the pipe work under the bath so i kbow the pipes are bonded.
Two walls are steel stud walls, so have bonded from pipework to the steel stud.
Does this sound right ??

 

[Midwest]

How can it be more dangerous than it was before ?

Because now, you have 'earthed' something that previously (potentially) was not connected to the MET, therefore is now a path to earth (there are others, who will described this in more scientific manner). If you feel the need to earth the metal pipe work (that may have not been extraneous), what about any metal single fuel towel radiator fed by plastic pipes, metal bath, aluminium windows?

Did you watch the video Pete posted, #3?

Edit; if you tested the metal pipework, and it was extraneous, then you have made the installation safer than it was.

 

[Wilko]

On the accident info ... I don't quite get why the tragedy occurred? The cable in the metal stud wall was penetrated by a screw (Sod's law) and the screw contacted the live only (Sod's law again) it made the frame live. Got it. But when the water first saturated the area and touched the steel frame the RCD should have tripped ? Or failing that when the person touched a bonded tap while standing in the live (but electrically isolated) puddle and provided a path then the RCD would have tripped? What have I missed ?

 

[Andy-1960]

On the accident info ... I don't quite get why the tragedy occurred? The cable in the metal stud wall was penetrated by a screw (Sod's law) and the screw contacted the live only (Sod's law again) it made the frame live. Got it. But when the water first saturated the area and touched the steel frame the RCD should have tripped ? Or failing that when the person touched a bonded tap while standing in the live (but electrically isolated) puddle and provided a path then the RCD would have tripped? What have I missed ?

Was there an RCD? Maybe the floor was insulated?

 

[Deleted member 26818]

No RCD, flat was wired to the 16th edition.

 

[Wilko]

No RCD, flat was wired to the 16th edition.

Thanks - makes sense.
Thinking about how this problem could have been picked up during an inspection - they imply as the chap was not qualified, he didn't inspect competently. But it seems possible (to me anyway) the circuit could have a compliant IR until the area was inundated, and so the circuit would have appeared normal ?

 

[Ian1981]

Having the protective conductors connected to Earth during an insulation resistance test the test has the advantage that it might detect any contact between a live conductor and any ‘unearthed’ metalwork (such as a concealed part of the building fabric or an isolated section of pipework). If the unearthed metalwork has some relatively low resistance contact with Earth (even hundreds of thousands of ohms), the defect can be identified by the person using the insulation test instrument. Such a dangerous defect might be caused by the penetration of a nail, screw or similar making contact with a live conductor and the unearthed metalwork. If not corrected, the defect would create a risk of electric shock.
Key word being MIGHT.

 

[Ian1981]

Found this extract online
The case relates to the Death of 22 year old Emma Shaw who was the occupant with her 18 child month of a rented 1st floor flat in West Bromwich in December 2007.

The flat was one of new 42 flats that had electrical installation carried out by Anchor Building and Electrical services an NICEIC Approved Contractor in 2006.

The internal walls were constructed of "C" section metal studwork covered with plasterboard. A hall cupboard contained a pressurised water boiler above which was mounted a consumer unit supplied from a distribution circuit from a switch fuse in a an riser cupboard external to the flat. The means of earthing was TN-C-S. None of the circuits in the consumer unit were RCD protected. Circuit No. 3 supplied one of the immersion heaters in the boiler.

The electrical installation had been 1st fixed the plaster board fitted to the metal studding before the installation was 2nd fixed. Circuit 3 supplied a 20A double pole switch at high level which supplied a cable outlet at low level, this in turn supplied the water heater in flex.

The cable to the flex outlet was longer than it should be and when the plasterboard was installed the cable was trapped between the plasterboard and the stud-work. In addition a plasterboard screw passed through the cable clipping the Line and CPC conductors and in to the studwork.

The installation had been energised without any testing and the fault current had blown away part of the screw and CPC. This was verified by forensic examination at the HSE laboratories. This left a high resistance fault from the line conductor to the metal studwork.

18 months later the stat. on the water heater failed and the cylinder over pressurised and the safety valve operated and the safety valve operated and discharged in to a tun dish. The plastic waste pipe from the tun dish had parted because it was not glued and the water spilled on to the floor soaking the carpet. The water soaked under the foot of the wall and in to contact with the live studwork.

Miss Shaw was mopping up the water and had texted her partner to come home telling him the hall was flooded and the "electricity was sparking". he partner tetxed back to say to turn off the water stop cock sited in the same cupboard as the boiler.

Later she was found kneeling in the cupboard slumped forward and apparently lifeless. Subsequent examination and a Post Mortum determined that she was kneeling in the charged water and received a fatal shock when she touched the earthed stop valve.

West Midlands Police, the ambulance service and the fire service attended the scene. The police commenced an investigation for a suspicious death.

The police later arrested both the QS Hoult and Tomkins the electricians mate and they were interviewed under caution. Tomkins had signed an Electrical Installation Certificate as the Inspector. He admitted he was not qualified or competent to do so. He said he had been taken to the flat by the electrical site foreman and told to do ring continuity and loop impedance testing.He said when he got to the flat the installation was already energised. He did no other inspection and testing. He said that in the site hut they had sat around the table with the site foreman and was told the other tests had been done and he was told what to write on the test certificate.

This EIC was submitted to Anchors offices where a type written certificate was produced. The type written version had differences to Tomkins hand written version. The typed version had "P/P C Tomkins" in the single signature box for design. construction and inspection. Tomkins said in court he had not seen the typed form and he would not have consented to his signature being used. Mr Hoult the QS said that he just checked the figures on the form and signed it as the QS. he did not go to site and was not involved in the testing and inspection. Mr Hoult had known Tomkins for many years and it was alleged he knew about Tomkins status as a mate and his lack of qualifications.

 

[Wilko]

None of the circuits were RCD protected, which must have been contrary in 2006 (?) and his signature has been put onto the EIC for design for the house. Then the Nice Guys didn't investigate the bang test properly which left the construction fault in place waiting.
Reading this, seems the fault may not show up on IR test during EICR unless the steel studwork is bonded (?). Also, if an RCD had been used the tragedy might well have been avoided. Appreciate any feedback ...

 

[Midwest]

I suppose if the initial energisation, had 'blown away part of the screw and CPC', then this may not have been noted on a L-E IR test?

However, the missing continuity (on CPC) should have been noted, in the very first of the dead tests, and investigated further.

 

[Midwest]

Incidentally, I was told of a similar fault a few years ago.

Whilst a domestic properties bathroom property was being refurbished, the plumber was getting an electric shock whilst touching a plastered wall (block construction) and copper pipe.

Turned out, landing light one of the strapper's wires in a twin & e, had been penetrated by a nail. Not touching the cpc, nor breaking the strapper wire. But made the whole wall 'live'.

 

[Ian1981]

None of the circuits were RCD protected, which must have been contrary in 2006 (?) and his signature has been put onto the EIC for design for the house. Then the Nice Guys didn't investigate the bang test properly which left the construction fault in place waiting.
Reading this, seems the fault may not show up on IR test during EICR unless the steel studwork is bonded (?). Also, if an RCD had been used the tragedy might well have been avoided. Appreciate any feedback ...

From what I can remember about the 16th edition was that cables in prescribed zones did not require additional protection buried in walls <50mm.
Circuits such as lights had a 5 second max disconnection time apart from a circuit in a bathroom which was 0.4 seconds and required supplementary equipotential bonding to pipework and all location circuit cpcs.
If it was assumed that no equipment could be used for outdoor use as was the case with a non ground floor level flat then omission of rcd protection of socket outlets may have been allowed. I'm not 100% sure of the omission tho unless others know for sure?

 

[Deleted member 26818]

None of the circuits were RCD protected, which must have been contrary in 2006 (?)

No, 17th was introduced in 2008.
No requirement in the 16th to provide RCD protection to anything other than sockets which would reasonably be expected to supply portable equipment outdoors.

 

[Richard Redman]

So, after reading through these feeds, am i understanding that a steel stud wall in a bathroom is an extraneous part, which doesnt need bonding as all the circuits in the bathroom are RCD protected?

 

[Ian1981]

Is the steel exposed?
If not then no as it can't be touched between this and an exposed conductive part.
Is it extraneous? Can it introduce earth potential?
If not then no.
If it's extraneous then it needs bonding as you would with the gas and water services.

 

[Richard Redman]

At present some of it is exposed due to the builder working on the other side of the stud wall, but once complete it wont be exposed. The fact the there will be a lighting circuit and socket circuit running through the stud wall, would this make a difference?

 

[Ian1981]

No both circuits will require rcd protection tho.
It seems the wall is just isolated metal work and will not require bonding.