UK 3 phase supply earth concern

[loldollard]

Hi all,

Background- I rent commercial premises for a small cabinetmaking business. Lots of 3 phase machines and 240v circuits. The site is on a farm where the main supply enters the landlords unit to a distribution board where it is then split to various rented units. That board looks like standard 3 phase board to me.

At my end I have an isolator, a meter then my distribution board which is 100A RCD protected.

The site has a good electrician maintaining it but my main concern is that my supply and earth comes from another unit which is not exactly tamper proof.

My worry is that someone poking around that primary board could compromise my earth and therefore my RCD protection - is this correct?

That unit does have a few 'old boys that know everything' and I'm sure people go to that board frequently to fiddle.

Sorry this is long winded but my question is should I consider this a concern for our workshop and should we be looking at installing our own earth rod or something to ensure earth at all times?

 

[Lucien Nunes]

I would say it's relatively unlikely that your earth would get disconnected, but how likely will depend on what type of cable is in use. If it is steel wire armoured connected to a metal distribution board at each end, it is actually quite difficult to not have a connection without physically removing the gland and cable. In any case one would have to be pretty careless and negligent to disconnect a random earth cable and leave it disconnected.

As for the RCD protection, the RCD will still function as an RCD without any earth connection via the installation. If you get a shock from line to 'real earth' then you will still cause an imbalance and the RCD will trip. What would not happen if the earthing is totally disconnected though, is a trip in the event of a fault in a machine that makes the machine chassis live. The trip will only happen when you then get a shock from the machine chassis to true earth or the machine or part of the installation contacts true earth itself.

 

[dmxtothemax]

WRONG !
Even if he completely dissconnects your earth line ?
Then the RCD will still function.

 

[telectrix]

WRONG !
Even if he completely dissconnects your earth line ?
Then the RCD will still function.

why should the RCD trip if the exposed conductive part that has the fault to is not earthed? unless the metalwork is earthe by virtue of being bolted to concrete floor?

 

[Jim_e_Jib]

If you have concerns, something like a Martindale socket tester will give an indication of the earth loop impedance at a 13amp socket in your unit.

It's by no means the best way to check this but may give you some peace of mind.

Perhaps you could put the kettle on and have a chat with the maintenance electrician next time they are on site?

Martindale EZ165 Advanced Socket & Non-trip Earth Loop Tester

Capable of detecting twenty-eight fault conditions An advanced socket tester as defined by the Health & Safety Executive Verifies earth wiring using a non-trip earth loop test

www.tester.co.uk

 

[pc1966]

Having a socket tester is a good idea for general use!

But you really should get one such as @Jim_e_Jib pointed to that actually tests the earth impedance. The cheaper ~£10 ones do a very basic functional test but will show earth with even a very poor connection (e.g. concrete resistance, etc) and you really need to look at the £40-60 region to get ones that do a proper test. Other examples are:

Kewtech Loopcheck 107 Socket Tester

Kewtech Loopcheck 107 Socket Tester - KT107 The Kewtech Loopcheck 107 checks the important parameters for safe wiring. It offers a cost effective solution to the users at the front end of responsibility and care. KT 107 Key Features: After being inserted in to a socket the loopcheck...

www.test-meter.co.uk

Socket & See SOK36 Earth / RCD Socket Tester

Socket and See SOK36 Earth / RCD Socket Tester The Socket & See SOK36 Check Plug is a fast and easy to use Socket Tester that ensures that 13A UK sockets are wired correctly as, indicated by all three high brightness, long life neon's illuminating. Potential faults are indicated by one or...

www.test-meter.co.uk

(Worth checking they are in-stock before ordering though!)

 

[loldollard]

Thanks everyone!

I do have a basic LAP socket tester but don't generally need much convincing to buy another tool!

Sounds like I am over thinking this but will definitely befriend the site electrician.