Type of Earth / earthing questions

[ntom]

I was just contemplating making a connection to a garden shed and was taking a look at my CU (s!!).....and had some questions regarding the earthing arrangements.

The incoming mains cable from eb looks like it is coated in a 'tar' cloth wrap & has a woven connector marked 'safety earth -do not remove' attached to it (just before it enters the main fuse) which is terminated in a connector box.

However there are no connections to this from the house circuits. I have 3 (!) CU units all connected via their own tails from a block which in turn comes off of the main incoming fuse. The 3 CU's are the standard house box (with 6 circuits) plus a separate dedicated cooker CU and a third CU which looks like it was added later providing a few extra 16A radial circuits in the lounge (& has a few spare outlets from which a shed supply could be taken).

Earth arrangements for all 3 CU's rely on connection / cross connection to the metal conduit which disappears into the concrete floor from all 3 CUs.

Q1: What type of earth is this likely to be?

Q2: Is it sufficient to earth via conduit, or should all 3 CUs also be cross connected & attached to the electricity board earth -it would seem safer in principal?

Q3: We are in the process of having a bathroom fitted & the plumber has laid (in part anyway so far) the cross bonding earth connections -will this go to the electricity board earth or the (one of!) CU units -does it even matter?

Q4: I thought that there was supposed to be a single switch off point (or is this a recent change in regs), or is it ok to have to shut off 3 switches to close off all power?

For info my house is in suburban surrey & was built in 1936. It has clearly been rewired as all cabling is t&e except for one of the ligting circuits which appears to be good similar age twin core (i.e. pvc) but uses a separate earth cable & (metal) conduit for earthing. I presume the incoming mains is as originally installed in 1936.

Thanks for any help!

 

[Lectrician]

Sounds like you have a TT arrangement. In which case, the CU's must have an RCD as the incomer, not a DP switch??

The earth to the EB sheath would be better, but it would need to be confirmed that this has a satisfactory loop impedance. This would be a TN-S system.

A single main switch should be present - but I loose count how many houses have an arrangement with several main switches.

Does the plumber know what he is doing - it's not his job to do the bonding. The regs have recently changed in respect bathroom supplymentary bonding. It is not as straightforward as it used to be.

 

[ntom]

Thanks for reply.

Hmmm.......not an RCD in sight. All three CU's have DP switch.

Plumber has left a coil of earth cable so presume electrician will connect this to board when he comes & does the shaver socket & lights. (For the time being) this is still on view.

 

[Lectrician]

The bonding from the bathroom shouldn't go to the main CU. It should be contained with in the bathroom connecting pipework and electrical points together.

 

[ntom]

so if there is no socket in the bathroom it is just used fror cross bonding of pipework? So this just ensures that all pipework is at equal potential....but not necessarily (in a worst case scenario) at earth?

Dont think there is any pipework in the house that is plastic anyway.....


Earth loop impedance test: if testing the incomer is this an eb job or can any electrician do it?

 

[mapj1]

Normally its enough to test downstream of the meters, i.e. from a short spur from the largest circuit breaker or fuse in the consumer unit. If need be one can be attached on the day for the duration of the test.

Essentially the tester measures the live-earth voltage, then when you press the 'test' button, connects a known load between live and earth (a resistor chosen to pass 10 or 20 amps, depending on design). The voltage step is then recorded. Strictly this measures R-live plus R-earth, but for deducing fault currents this is what you wanted to know anyway, otherwise do a similar L-N measurement and deduce how much of the step is really live feed resistance, and how much the earth. In reality, on a TT system the earth resitance dominates, being 10 to 100 ohms, and on PME or TNS the value is usually so low, perhaps 1.3 of an ohm, that it is neither here nor there, so long as it is low enough, as final circuit wiring resitance will dominate, being ohms..

If you contact your DNO, they will probably say 'we dunno' but really should, when pushed, and reminded of their statutory duties under the 200s supply act, be able to tell you what earthing arrangement and impedance you have been given.
see here ...http://www.pv-uk.org.uk/reference/grid_con/dno_contacts.html

They may believe there is a 16mm earth tail between their existing earth tail plus clamp and an appropriate earth block on the wall nearby, or sometimes on the meter board.
like this.... http://www.tlc-direct.co.uk/Products/TLEB8.html

with wires from there to the CUs, the plumbing and gas etc, but in reality it may have never been fitted, or came off at some point, and should be replaced, or there may be a good reason not to use the company earthing point, due to a problem with the underground cable connections. In any case they should know.
Otherwise if they say 'TT' you need an earth rod in wet earth feeding that non-existant earth block, and an RCD as a matter of urgency.

 

[mapj1]

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