Sub Main Earthing

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davetheglitz

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Me and another sparky did an inspection today on a house converted to 7 bedsits. Each bedsit has it's own submain to a pay meter and RCD protected DB. Each bedsit also has water services.

The whole place was a shambles and is in need of a rewire.

We had a debate about how each DB should be earthed/bonded.

One arguement is that each DB should have a substantial earth wire to the MET (10mm G/Y + cpc from 6mm T + E) to ensure bonding/earthing was adequate as PME system earths should be able to take the full neutral current in the event of neutral disconnection.

The other arguement was that if this was a house you wouldn't even be having the debate for each room. The bonding is mainly to protect from an external influence. Main bonding at source should allow for this. If a plumber decides to fit a lot of speed fit connections the 'far end ' of the plumbing installation will only have to cope with problems caused within the house - so 2.5mm supplementary bonding to water at each DB would be adequate.

I tend towards the second arguement - but I would be interested to see if I'm on my own here!

One interesting comment was made in our discussions. Britain is the only country that uses PME - no-one followed us for good reason! Now we have the 17th with the RCD protection on everything (nearly) electricians to a man should be getting rid of PME where possible and installing earth rods! I'd be intrigued to know what other people felt about this - particualrly Mr Steptoe knowing his Kiwi roots!

Cheers

Dave

 

steptoe

of course Im wrong, ask my wife™
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thanks for the prompt sir. ?:|

as i see it this all depends on what would be classed as a dwelling.

IMHO I would tend towards each bedsit being as its own individual installation,

although, if each has its own fused isloator at the main incomer i see no reason an MET couldnt be placed there and then one main bond taken to the incoming water/gas etc. as in a residential home for the elderly etc.

the only proviso to this being if there is separate metering withing each unit, or its own stopcock/isolator for the water/gas etc.

as for your 2nd point of PME,

I do regard it (as do most of the world, I think,) as a bit of a major joke.

even Ireland, North and South dont rate it much, and its only really to be seen much in towns where it is convenient(read cheap!) to install.

PME has been known to lose a N,.! :eek: thus rendering everything earthed to a potential 230v , this is why IMHO the regs are now starting to backtrack to using RCD protection for everything, and soon they will also make it virtually impossible to install a ring final circuit(having a cable capable of carrying 18a protected by 32a MCB) .

all of course in the name of better/improved safety and not in admitting that they have been wrong for all these years and are now finally following the majority of the civilised world.

ie, 20a RCBOs with 2or3 sckts per circuit with no inconvenience to other circuits if it trips out.! ; \

back to your original point, if there is no isolator present at point of origon you could always fit one and treat each bedsit as a radial from the origin.

hope Im making sense,

beer+lack of sleep=me talking rubbish(or is that sense?)

HTH, Albert. :x

 
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thanks for the prompt sir. ?:| as i see it this all depends on what would be classed as a dwelling.

IMHO I would tend towards each bedsit being as its own individual installation,

although, if each has its own fused isloator at the main incomer i see no reason an MET couldnt be placed there and then one main bond taken to the incoming water/gas etc. as in a residential home for the elderly etc.

the only proviso to this being if there is separate metering withing each unit, or its own stopcock/isolator for the water/gas etc.

back to your original point, if there is no isolator present at point of origon you could always fit one and treat each bedsit as a radial from the origin.

hope Im making sense,

beer+lack of sleep=me talking rubbish(or is that sense?)

HTH, Albert. :x
You usually make sense......

Evenin` All.

I tend to concur; esp. as regards the main isolator at incoming position.

 

davetheglitz

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Thanks guys - in this case each bedsit would be fed as a radial from a 40A MCB at the main CU, with a 20A and 6A MCBs at the room DB protected by a 40A 30mA RCD. There is only one main water bond.

We also need to investigate getting another phase in as each flat has a small cooker or oven and there are two 9.5kW showers in the property. Diversity - whassat!

If I'm reading the replies correctly this should be adequate - and I would suspect it is probably a good idea to do some supplementary bonding. The additional 10mm earth would not be necessary in this case.

Fully agree with your PME concerns Albert. When I was in Spain recently the MCBs I saw were actually double pole - so fault finding would be a breeze. I find it sad (mainly that I was even looking when I was on holiday!) that we haven't started to go this way.

Dheers

Dave

 
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