Earthing isolator switch

[Sparky Jim]

I don't make the rules Guys, so please do not shoot the messenger. I personally think it irrelevent, but they say it is required. I believe they are interpreting Regulation 544-01-01 in order to state what they do. I can't see a flaw in thier reasoning except it flies in the face of common sense.

sparky Jim, i thought NICEIC state 100 mA time delay up front 30 mA socket side of split board for a TT supply?????

Why are you stating it was a TT supply, I did not state what type of supply we had in the premises, but for your information, it is a TN-C-S (PME) supply with each service being rated at 125A.

Regarding what the NICEIC state, they do not make the rules anyway, so I care little for what they say. They can make recommendations, and I will listen, but at the end of the day, I will install what my Boss and the Consultants spec for the job at hand.

 

[Lectrician]

Why an RCD protecting the whole install for a TN-C-S supply??

I think the reg you quoted having a baring on this is a farse! Not on your behalf, but who ever is banging the line to you.

I will be taking this up with the relevent people, as I have had work inspected also, as a matter of course, with no probs at all.

It is pure lunacy to put an earth of high impedance in close proximity to a live conductor fused as highly as that at a service head.

I understand you are 'messaging' what you have been told, and have no problem with you.

 

[Zoonie]

The only time that smaller bonding cables are used is for supplementary bonding, and these can be 4mm if mechanically protected ie: installed in conduit or trunking, or 6mm if they rely on their sheath for protection. These cables tend to be used for cross bonding of pipework and other extraneous metalwork in the installation that could concievably become live under fault conditions.

Am i behind the times i thought it was 2.5mm if mechanically protected or 4mm if they rely on there own sheath for protection.

 

[paulh53]

Am i behind the times i thought it was 2.5mm if mechanically protected or 4mm if they rely on there own sheath for protection.

Ditto Zoonie That's what I thought too

 

[Scoby_Beasley]

Hi - Can the panel advise on the following please:.........

There is no panel mate, just good people with good hearts trying to help others

 

[Sparky Jim]

Call me old fashioned, but I always use 6mm for cross bonding, it is what i learnt my trade using. Yes I know this has been downgraded by the changes made to BD7671, but hey, old habits die hard.

Why an RCD protecting the whole install for a TN-C-S supply??

I think the reg you quoted having a baring on this is a farse! Not on your behalf, but who ever is banging the line to you.

You may have a point, but to be honest I cannot say i have given that much consideration, simply doing what I am told.

I will be taking this up with the relevent people, as I have had work inspected also, as a matter of course, with no probs at all.

Whatever floats your boat, personally I have far more important things to do with my life than this, it is not what I am paid to worry about.

It is pure lunacy to put an earth of high impedance in close proximity to a live conductor fused as highly as that at a service head.

Why do you consider the Earth of a PME system to be of high Impedance?

I understand you are 'messaging' what you have been told, and have no problem with you.

Whatever.

 

[Lectrician]

Why do you consider the Earth of a PME system to be of high Impedance?

I am talking about a TT system, which is what this topic was about.

As a TT earth is bound to have a high impedance, earthing the metal enclosure of an isolator that is before the RCD is dangerous - yea, the fault current can flow, but it will be now where near high enough to provide disconnection of the fuse in the service head.

 

[Sparky Jim]

OK Lec, I understand the comment about High Impedance now, if you'll pardon the pun, we appear to have got our wires crossed

I see where your coming from with the concern regarding the enclosure, however if the protective device were a time delayed RCD proporly rated for the installation and role it will play in it, then there should not be an increased risk of shock.

Lets use this TT system as the basis for this discussion.

We have the Head, probably with a 100A BS1361 Fuse inside, the Main Service Bond is via an Earth Stake in the grounds of the premises and this currently connects the Stake to the Consumer Unit, now lets introduce a New Consumer Unit and update the installation for compliance with BS7671:2004.

As per Regulation 542-04-01 we shall install a Main Earthing Terminal and from this I shall install a New Main Equipotential Bond to the Earth Stake, which I would test initially to ensure compliance and where possible reduce the impedance of the stake. This new Bond would be 25mm 6491X single core copper cable. The Regulations allow the use of 16mm, but the reduced impedance of the 25mm will increase safety slightly, and all gains that can be made in a TT system are worth every effort.

From the Main Earth Terminal, I would then run a 25mm Bond to the Consumer Unit Main Earth Terminal, again to keep the impedance as low as possible. The Metal Enclosure you mention I would install unless absolutely unavoidable, I would install an Insulated unit, however I would install a 25mm bond from the Earth Terminal in the enclosure, metal or insulated, back the the Main Earthing Terminal. Also from the Main Earthing Terminal I would install 16mm Bonds to the Water Service and Gas if any existsed in the premisises.

Now for the Main setup. The Consumer Unit I would install would be a split load unit, very similar to what we have installed in the installtion I spoke of in my earlier posts.

All sockets and power using circuits would be protected by a main switch with a 80A 30mA Time Delayed rating. On the other side of the board I would install a 63A Isolator feeding ONLY the intieror lighting circuits of the installation. These lighitng circuits being protected by Type B MCB's of a rating as near to the design current of the circuit as is possible. In a small rural property it is possible to install a 3A MCB for a Lighting circuit due small number of lights on the circuit. Simply installing a 6A as it's what we normally do is not always best practice.

The Isolator to be installed between the meter and the Consumer would would be a 500mA 100A Time Delayed RCD.

I am aware that many will say that the Regs say the Main switch should be a 30mA RCD, however all truly vulnerable circuits are protected by such a device in the main consumer unit, and further, such an installation does comply.

My personal thinking is that putting lighting circuits on RCD's is potentially more dnagerous than not putting them on it. The chances of a person getting a fatal shock from a lighting circuit are much lower than the chances of the main RCD being operated by a minor fault on another circuit or more likely nuisance tripping from other influences.

In such an installation I have always ensured that any socket which may be used to feed externally to the building are MK RCD protected units, thus reducing the risk of main switch tripping even further, as it is often lawn mowers etc that cause such faults in domestic premises in my experience, yours may be different. Further on TT systems, which are abundant where I live, I have never allowed an Earth stake to have a reading above 50 Ohms. I know that the Regs say a much higher figure is permissable, but I feel happier with this reduced figure.

This is my take on things, I appreciate others may disagree.

______________________
moderator

Sparky Jim, please note 10a

 

[Lectrician]

Apart from the cable sizes being OTT, thats sounds fine.

This topic is about a TT install with the RCD integral to the CU. The ISOLATOR (not RCD) would be in the enclosure.

The difference of impedance between 16 and 25 is negligable when compared to a rods impedance, even if this were as low as 50ohm.


I do not understand why a time delayed 30mA for the sockets??

The way you describe the CU implies the main incomer is a 30mA RCD, and the 'split' part of the board has a DP isolator?? I may be reading this incorrectly??

 

[paulh53]

Call me old fashioned, but I always use 6mm for cross bonding, it is what i learnt my trade using. Yes I know this has been downgraded by the changes made to BD7671, but hey, old habits die hard.

It's been a minimum of 2.5mm2 (Mechanically Protected), Or 4mm2 (Unprotected) since the 15th edition IEE Wiring regs, (Donkeys Years)

 

[Sparky Jim]

It's been a minimum of 2.5mm2 (Mechanically Protected), Or 4mm2 (Unprotected) since the 15th edition IEE Wiring regs, (Donkeys Years)

PaulH53, it was reduced as an ammendment during the 15th edition, when I did my apprenticeship it was 4mm protected and 6mm unprotected. If I remember correctly it changed to the smaller sizes when I was working abroad in the late 1980's.

 

[Sparky Jim]

Moderator, what is that load of old tosh under my post above, what you have posted does not make sense when I read the guidelines. Would you be so kind as to post why you have left that message?

 

[mapj1]

Jim,
If he doesn't post, dont take it personal - I think all of us who hang around here have been flamed at some point about the club rules , and left slightly bemused wondering what we did wrong.
Coming back on track I'd agree with the use of a boskier wire though - not really a matter of impedance if the rod is 100R and the wiring 100milli or less, but because in a lot of agricultural properties it gets a pretty rough life, and anything that reduces the chance it getting the chop prematurely must be good. Sorry if I was stating the obvious earlier - I wasn't aware of your pedigree (Though I still think leaving a TT earth out of the plastic box with Pre-RCD live is better really, whatever the boss says )
One of those special cases the reg-writers never really forsaw being included I suspect.

 

[Sparky Jim]

No problem Mapj1. I guessed the Moderator was a bit like a "Willo the wisp".

I think what we all forget sometimes is that the Book (BS7671) is only a minimum Guideline and that we who are professional have a moral and professional duty to assess each and every job we do and then make decisions as to how we undertake that job. All work should always comply with the book, unless there are very good reasons for deviation that have been confirmed by an engineer. If we feel that it is better to exceed the requirements of the book, then it is not wrong to do so, in fact I feel it should be encouraged. What are we teaching the youngsters if we ignore the book when it suits us, or do the bare mnimum to comply. If we want them to be professional, then we have a duty to show them how to be and equip them with the skills and the menatl tools to maintain that professionalism.

 

[theshogun]

With regards to mapj1 earth in a pre rcd tt installation the regs do support this
read bs7671
reg 531-04-01 in conjunction with reg 413-03-09