How can I test the earth cable is actually earthed?

[efj]

Hi,

Was going to replace a plastic light switch (1 bulb on/off) with a chrome version. The existing cable into the light switch is 2 core and earth so it should be easy to connect the earth to the back of the switch plate.

However, when lifting floorboards I found the following junction box type thing..

URL http://picasaweb.google.com/elliottfjones/Electrics/photo#5036157111535113538

It looks like most of the lighting cables for upstairs feed into it and some, but not all of the earth's have been loosely wound together. So my question is how I can test if the earth cable feeding into the light switch is actually connected/earthed?


Cheers

Elliott

 

[plugwash]

you can use your multimeter to test for continuity between the earth on an extention lead plugged into a socket and the earth at your lightswitch position, this will tell you if there is some form of earth path back to the CU.

however what it will not tell you is whether the CU earth is actually properly earthed, nor will it tell you if your earth connections are relying on a distinctly unreliable twist like the one you have photographed.

imo when you see stuff as bad as this the only safe thing to do is to expose and inspect all the wiring in the floor and ceiling voids of the property and if you see anything amiss with regards the number of wires going into walls VS the number coming out at the fittings expse them too.

 

[Adam_151]

The standard way of proving the cpc back to the fuseboard is an R1+R2 test, you join the phase to the cpc at the board, go to furthest point on circuit and measure between them with a low resistance ohm meter and this gives R1+R2 value to be recorded on certs.

I suppose you could do the same thing with a cheap multimeter if you just wanted to prove that it was there, and weren't after an exact value... not the proper way to go about things obviously, but better than nothing.

Later on in the squence of tests, the earth to the fuseboard is tested using an earth loop tester, and then the loop tester can be used on the individual circuits once you have proved Ze and and R1+R2 separately... If you are in a rush you might be tempted to dive right in with the loop tester on the circuit, but if there does turn out to be a break then there is a risk of electrocuting people (though some would argue its better for them to get a short shock from a loop tester while there is someone on site, rather than a shock from a fault with no limited duration when there isn't, but thats not really the correct POV and doing the tests in the correct order is even better!)

 

[plugwash]

yeah thats what you'd do if you want to measure the r1+r2 but its overkill for just proving an earth is there and an ordinary multimeter won't give you usable values for resistances that low anyway.

however i'd still maintain that after seeing something as bad as that box testing alone without inspection of what is in the floor/ceiling voids would not give me confidance in the safety of the rest of the installation.

 

[efj]

Gents,

Thanks for the replies. I'm think getting someone in is probably best as there are a number of things I'm unsure of - I aslo need someone to test the upstairs ring main (as per my other posting).

Thanks for your help.

Cheers

Elliott

 

[Steve]

can you open that box up and post another picture? I bet its a real rats nest in there, looking at the additions etc.

 

[tim west]

Hire an earth loop impedance tester, only reliable way of checking the total earth path back to source.

 

[plugwash]

Hire an earth loop impedance tester, only reliable way of checking the total earth path back to source.

such testers certainly have thier place as a final check or to get some information on parts that can't be tested any other way (such as the incoming earth from the supplier)

however they aren't very accurate especilly when used in thier low current rcd-safe modes and they give the overall value back to earth including the affects of paths that cannot be trusted to remain intact.

but most importantly NO TESTER WHETHER MULTIMETER, LOW OHMS METER OR EFLI TESTER WILL DISCOVER A CONNECTION THAT IS CURRENTLY INTACT BUT MADE IN A WAY THAT IS VERY PRONE TO COMING APART. Given that such a connection has been discovered hidden in a floor void the only responsible thing to do is to assume there are probablly more and perform a thorough inspection of the wiring in the floor and ceiling voids.

 

[Adam_151]

Though not a total answer... a loop tester on the full 25A test range would go someway to proving the earth isn't hanging on by a single strand and is going to burn off at the slightest hint of a fault

 

[tim west]

they give the overall value back to earth including the affects of paths that cannot be trusted to remain intact.

Of course regular interval testing is a must. but hopefully the Electricity board will make sure their side of things fall within specs (fingers crossed).