Earth Protection

[PrenticeBoyofDerry]

I would prefer the total safety of everything rcd protected and risk full blackouts rather than any of my family being on the receiving end of a live fault that doesn't disconnect.

The requirements of BS7671 (wiring regulations) do ask for circuits to be arranged so this is prevented.

I expect it would be expensive, but is there any reason why you couldnt fit a rccd or rcbo on each circuit to keep the protection level while not risking total loss?

You can fit RCBOs to each individual circuit depending on the consumer unit you purchase, the price of each RCBO will vary between £15-£35.

The other options are (sorry if covering or repeating already given info):
Dual RCD which will cover two banks of MCBs, which will allow you to dived circuits so power is available on either side and if one RCD trips the split side will offer power for lighting, either by sockets or the lighting circuit.
In well designed install lighting and socket circuits can be split, so both are available on each split of the RCD, if one RCD trips.

You could also consider boards that offer both the facility for RCD protecting a bank of MCBs and individual circuits protected by RCBOs.

If you are considering an upgrade to a new RCD protected board whether it be by RCDs or RCBOs, it is advisable that you have your existing installation checked out via a electrical installation condition report, prior to the change over as suitability most be confirmed and any faults that could cause and imbalance in phase and neutral most be eradicated first.
Also there may be the possibility, that even split lighting circuits in older properties have shared neutrals.

 

[ebee]

Purely electrically speaking, having an RCD should be safer than not having one.

In truth, a few years ago when RCDs became vogue, the only option was a front end RCD built in the consumer unit as the main switch (or a RCD incomer on the tails before the mainswitch giving the same result) .

I, along with thousands of others, am a "Front Ender".

Often not troublesome but in theory can be.

When the "split board" came in the split was for some RCD circuits and some non RCD circuits therefore only some circuits on a common RCD, the remainder being either non RCD circuits (just a plain MCB) or RCBO circuits (each circuit having its own combined RCD & MCB).

When the dual board came in (often referred to wrongly as a "17th Ediition Board") then the split was two banks of circuits each bank collectively connected via its own RCD.

Another variant of that is the so called "high integrity" consumer unit (two banks of seperate RCD common connected circuits plus another bank of non RCd circuits for either MCBs or RCBOs).

The ideal solution might well be a plain switch and all circuits with RCBOs,
traditionally a very expensive solution but nowadays RCD & RCBO costs have reduced significantly so worth a thought.

Usually, RCDs to replace main switches are Double Pole switching wheras often single module RCBOs are only single pole switching (They do not disconnect the Neutral although they will still detect and trip on a N to E fault fault - continental models tend to be DP switching).

on a TT system then all lives (Including the N) must be disconnected when working upon, so DP RCDs or RCBOs are better (although you could argue that any DP mainswitch before it fulfils that function ).

Similarly if you are using an RCD/RCBO with an upstream RCD/RCBO (usually time delayed and of less sensitivity to achieve discrimination) then DP should be used to disconnect the N too.

Sorry to waffle on.

Back to the original point.

It sounds like you have been misinformed or have misunderstood. Although an RCD might well help to mitigate a poor earth connection on any TN system whilst this is being investigated/corrected it is not needed purely because you have any particular TN system.

 

[bernardgreen]

In truth, a few years ago when RCDs became vogue, the only option was a front end RCD built in the consumer unit as the main switch (or a RCD incomer on the tails before the mainswitch giving the same result) .

Thirty years ago I fitted two RCDs and the meter installer refused to connect as having two RCDs broke the rules. I explained to his boss that one was for the building site ( DIY house build ) and the other for the caravan so after an accident on site that tripped the RCD we would still have lights for first aid the boss told him to install and connect.

That was in the days when common sense could be used.

 

[ebee]

"That was in the days when common sense could be used."


Gosh Bernard, I think it was a lot more than 30 years since those days were upon us!

LOL

Sadly you don`t surprise me.

I actually reassembled a consumer unit to reconfigure it for two RCDs (what we now call a "17th Edition" ) labelled it all up accordingly. This would be about 12 years ago or more. 18 months after a "Periodic" was done and this was faulted as "Two RCDs in consumer unit code 2" .

LOL .
Incidentally another gem was "Downstairs socket on upstairs ring" - I had labelled all circuits correctly making it clear which sockets belonged to which circuit, there was no upstairs/downstairs mention of circuits either at the CU or the Schedule next to it but the "inspector" had noticed from that schedule that three outlets in one room were on one circuit and one outlet was on another circuit (all detailed ).

Mind you I`d seen scores of "Periodics" by that contractor that were laughable to say the least.

 

[RF Lighting]

Depends what you class as 'shot'

Case in question: Earthing Conductor has been cut back due to damage or corrosion, hard to say which. So the earthing conductor at present is about 30mm long and stripped bare of the incoming supply and extending using MET to fuse box. The Earthing conductor is a solder joint to the incoming supply sheath.
Ze is coming at 0.06Ω

If it's an old cast iron cutout with a 2.5mm² earth then I would expect them to upgrade all that equipment for free as well.

It's 6.00mm and the metering company and DNO have quoted in the region of £160+VAT to upgrade.

If it's just tatty then they might, they might not. It will be upto the DNO engineers discression.

I refer to the above information

They have offered me PME but why would I prefer a PME to a TN-C as the Ze is generally lower?

Sounds like there's nothing wrong then if the DNO want to charge for an upgrade.

The Ze is low enough and 6.0mm² just about squeaks through on the adiabatic to operate the suppliers fuse, so from their point of view the installation is a little tatty but safe so any upgrades are down to your customer to pay for.

Personally I'd leave it alone, as all you'll get for your £160 is 200mm of 16.0mm² braided earth wire, a constant force spring and if you're lucky a SP henley block.

PME (TN-C-S) will generally have a lower Ze with a maximum of 0.35Ω than a TN-S supply which can be as high as 0.8Ω

As your supply has a 0.06Ω incommer there is no benefit in changing the supply to PME

 

[PrenticeBoyofDerry]

PME (TN-C-S) will generally have a lower Ze with a maximum of 0.35Ω than a TN-S supply which can be as high as 0.8Ω

They seem to be the generic values given, but the area/location I work in. It seems the opposite to be true.
Hardly ever get a TN-S measuring above 0.20Ω
and very rarely get a TN-C-S measuring as low as 0.20Ω.

As your supply has a 0.06Ω incommer there is no benefit in changing the supply to PME

But in this case, I suspect sleaving the earthing conductor (might be a little over 30mm in length but not much more) Then terminating back in to MET block, then extending to board in 6mm, 10mm or even 16mm for a distance of about 600mm will give very little difference in measured values.

 

[RF Lighting]

PME (TN-C-S) will generally have a lower Ze with a maximum of 0.35Ω than a TN-S supply which can be as high as 0.8Ω

They seem to be the generic values given, but the area/location I work in. It seems the opposite to be true.
Hardly ever get a TN-S measuring above 20Ω
and very rarely get a TN-C-S measuring as low as 20Ω.

Are you sure those figures are correct? Your lights would be incredibly dim if you had a PME supply with a Ze over 20Ω!

As your supply has a 0.06Ω incommer there is no benefit in changing the supply to PME

But in this case, I suspect sleaving the earthing conductor (might be a little over 30mm in length but not much more) Then terminating back in to MET block, then extending to board in 6mm, 10mm or even 16mm for a distance of about 600mm will give very little difference in measured values.

You shouldn't sleeve the DNOs conductor. Being a bare conductor will have been worked into the adiabatic equation. You should then take a 16.0mm earth from the MET. It probably won't affect your Ze reading, but it'll leave your installation safe.

 

[PrenticeBoyofDerry]

Sorry RFL edit 0.20Ω

 

[ebee]

Around here PME is often 0.16 to 0.22 and TNS 0.2 to 0.24.

What you gotta realise though is that whilst bonds in you installation are removed to test but bonds in nearby installations are still in place and are often pulling the E down.

I suppose that without neighbouring bonding we might find 0.35 and 0.8 figures more realistic maximums

 

[ban-all-sheds]

I actually reassembled a consumer unit to reconfigure it for two RCDs (what we now call a "17th Edition" ) labelled it all up accordingly. This would be about 12 years ago or more. 18 months after a "Periodic" was done and this was faulted as "Two RCDs in consumer unit code 2" .

That would be excellent news if you had commissioned the PIR for your own use, as it would mean that you wouldn't have to pay for it.

 

[ebee]

You know when you think (off top of your head) about 12 years ago then check and it`s 15 or 20 years ago? well it wasn`t ! - LOL - it was between July & Sept 2000! I guess tyhe "PIR" was about 18 months after that.

The installation was one of two I did like that that year.
Tails, DP Isolator, Henley feeding two pairs of tails to 2 RCD "Mainswitches" in one Consumer unit , each RCD feeding about half the MCBs, split sensibly and fully labelled on the CU and Main Isolator and circuit schedule stuck adjacent to CU and warning about complete isolation via external isolator before gaining access inside CU.

Price Charles got involved with that one!
No I don`t mean he helped me install it!
On one of his walkabouts he was pictured sitting in the Ladies lounge next to a socket I had installed over 18" from finished floor level to underside.

You see I was ahead in socket mounting heights too!

Ban - I`m not too sure what you mean - could you expand on that please?