1 Earth for 2 supplies

[steveth]

Hi, Ive come across an installation that doesnt seem quite right, ill get my red bible out later and have a look what i can find but hopefully someone on here may offer an opinion. Ill try and be as clear as possible. I realise its not a DIY question but maybe someone on here has come across a similar situation before.

It is a factory setup, it has 2 x 400A three phase supplies. Each supply consists of an SWA cable (looks like one anyway) coming up out of the ground and into a DNO metering panel( the swa must terminate inside the panel somehow as its not glanded into the bottom of the panel, it just goes up through a hole in it). From the panel comes 4 double insulated 'tails' cables (L1,L2,L3 & N) that go into a 400A TPN switchfuse which act as the main switch (the tails look like 185mm). This setup is same for both supplies........

now, here's the bit im unsure of...... On the side of meter panel 2 is a bolt protruding from the side. from this bolt a 70mm earth is taken to the 2nd switchfuse earth stud (the earthing conductor then) and also to the 1st switchfuse earth stud, then to a bolt to a local steel beam which is where the equipotential bonding also joins. There is no earth connection to the 1st meter panel and all this equipment is mounted on a concrete wall (so its not all sharing a common metallic frame).

To me it looks like both supplies are sharing supply 2's earth path back to the substation (presumably the armour). My question is would this be allowed? i suspect not and that each supply should be earthed individually even though they will join at the MET( which is currently just the bolt through the local steal beam).

opinions welcome, the earthing conductor also doesnt seem big enough to me at 70mm but one thing at a time eh,lol. system is TNS and the 'ze' is excellent (0.05, no pp's disconnected though)

 

[rssteve]

i can;t remember the name but some big places i have visited have 2 seperate supplies into the same panel, there is then this thing(name?) between then so the supplies can be joined together maby if one supply is lost etc.

 

[sparkyspike]

The main earth conductor needs to be able to take the fault current of both installations (unless they are separated from each other which they're not). Does the supply armour terminate at the main earth stud? The 70mm looks only good enough as structural bonding which means you have a main earth conductor deficiency between your installations and the supply armour? Have you looked inside the meter cabinet or the switchfuse?

 

[NotHimAgain]

Some quetions for you:
1) Is there any evidence of an earth conductor from the sub-station, such as a separate black single core cable?
2) Does the sub-station just serve this installation and no other?

I am trying to establish the earthing system. If the answers to questions 1 & 2 are no and yes you will probably find that your 70mm² earth is connected straight onto the neutral inside meter panel.

This is called Protective Neutral Bonding (PNB) - it is similar to PME (TN-C-S) but it is only used for a single consumer, that is, no other consumers will share the connection to the sub-station.

On the fault current issue - remember that simultaneous faults on the two installations would not produce twice the fault current. Fault current is limited by supply system impedance, and in this case that would be mainly due to the local supply transformer. You could get a small increase due to the lower supply cable impedance (two in parallel for simultaneous faults) but this is unlikely to be very much.

 

[steveth]

the substation supplies other installations too, of that im sure, there are a couple of other factorys on the street and a couple of houses.

there are no other cables coming up out of the floor. the setup as i said is

swa-------meter panel---------switchfuse. this is the same for both supplies which arent linked.

swa supply cable goes straight up into the meter panel of each supply (not glanded, just through a hole in the bottom) from the top comes 4 singles cables (red/orange colour, probably double insulated) that i would estimate as 185mm, these are identified n, red, yellow, blue. they connect to the switchfuse as expected.

what makes me think one supply isnt earthed is that a 70mm cable goes from a stud on the 2nd meter panel to both switchfuses and to the MET. the other meter panel is seperate from everything else completely (ie not all mounted on metallic frame). the only thing connecting the meter panels is a cable that looks like a 3 core 1.5mm swa (i presume this is for metering purposes?

cant get access inside the meter panels as they are sealed.

 

[NotHimAgain]

From what you say it is unlikely to be PNB, but it could be PME (TN-C-S).

It looks like the only way forward is to contact the DNO (best of luck ) they are required to give you information on the earthing system but don't hold your breath.

 

[steveth]

i reckon its tns myself but we shall see...... ze is only 0.03 but then again the transformer is only 15 metres across the car park!

got in contact with the DNO who 'dont hold' records, when i reminded them they had to tell me the information i require under section 28 of ESQCR regs (i think it is) they said we'l send an engineer round to have a look for you........in 6 weeks.

reason i think its tns is because surrely when the new supply was run in the DNO had to have a hand in it, they wouldnt have allowed a 70mm earthing conductor for a TNCS i would think and they probably would have to have been present to seal the meter panel after earthing conductor was connected to there meter panel. then again..........

think this work was done around 1992, dont know when PME started becoming popular, there isnt a PME sticker anywhere to be seen either which i believe should be present.

 

[holmslaw]

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