SITUATION - I have an old Wylex CU which has had the wire fuses replaced with MCB's some 12 years ago. The CU itself must be years older. Doing some work in the cupboard I noticed that there was no earth cable coming from the CU's earth terminal to an external earth block which in turn is connected to the steel armour of the incoming cable [45mm diameter] [TN-S] system I believe. My other separate Shower CU is OK and has an earth cable connecting to the earth block. Also connected to the eath block are the bonding cables and earth cables from water and gas pipes etc.
I opened up the CU and the only contact between its earth terminal strip and any sort of earth was a small [3 mm] cable running from the earth terminal strip into the incoming main fuse/isolator switch and from there to the steel casing of the main garage switch which has a steel conduit to earth.
WHAT I HAVE DONE - As a temporary measure until I can get the CU replaced I have connected the CU's earth teminal strip to the Earth Block with 16mm arth cable which in turn is connected to earth via the incoming cable armour.
QUESTION - Have I made things better or worse?
You have made things better of course if I read you right. 16mm2 E conductor from MET to service head all good. Obviously its 10mm2 from water and gas and oil and the like to MET or E block, 16mm2 from MET to E block and onto armour as per regs.
Jackhawk- Many thanks for yur response.
As a matter of interest the Isolation Switch I referred to is a Crabtree 500Volt, 60Amp, 50 Herts Safety Trip Switch with a manual override and test button. It is located after the the Electricity Board's meter and Main Fuses.
Is it possible that the earth connection from the CU earth Strip to this Switch is intended to trip the switch in fault conditions and shut down the supply?
It has never tripped since we have lived here [13 years]
With older systems, TT mainly, a 100mA RCD would be installed between meter and CU. In the event of a earth fault in the installation it would trip. The down side would be total darkness till you got to your CU to reset it. As you say you have a test button (which only tells you the the machanical side works, not the fault detecting side) I would say it is a RCD of sorts. As old systems are upgraded from TT to TN-C with meter changes, they only do meter backwards, so all from meter to your CU is yours and not their problem. Sometimes with a meter change they will install a DP switch between meter and CU. Not often though.
The device you refer to is the forerunner of the RCD and is in fact a voltage operated earth leakage circuit breaker.
This device MUST have all earth circuits directly connected to it in order to operate.
There would have been a small earth wire from either the incomming main cable or earth rod to one connection on the trip.
the other earth connection would have gone to the fuse boards earth terminal strip, to where any other earths should have been connected.
The device worked by measuring the actual current passing through the earth path.
So by putting ANY earths direct to the incomming earth will bypass the trip and render it inoperative!
By comparison an RCD measures the current going out in the live & returning in the Neutral and assumes any difference must be due to leakage and requires no earth reference,
so at the moment your main fuse board has no effective earth leakage protection and that CU change should be a priority job,
Following my last question I am a little worried and have one more question regarding this system of mine. As I will not get anyone to do it before Easter now.
From the Crabtree Trip Switch the leads go to a Splitter Box and Splits 3-ways. One set of cables go to this old Wylex CU as described before. A second set of leads go from the Splitter Box to a New 2-way 63 amp RCD Shower CU for the electric shower with its 16mm earth lead connecting to the Armouring of the incoming supply cable. via an earth block. No connection to the Crabtree Trip Switch.
The third set of leads go to the Garage Main Isolation Switch located in the same cupboard and connected to an RCD CU in the garage 30 metres away via a armoured cable and is earthed via an earth Rod at the garage.The Earth lead from the Crabtree Unit is connected to the steel casing of this switch.
My question is this- Since the Crabtree Switch is connected to my old CU's earth terminal and then from the Switch to the steel casing of the garage main switch does it affect the shower CU safety and the Garage CU apart from shutting them all down if it trips?
I would leave things as they are as I think the shower installation probably did the by-pass bit some time ago!
At least you now have a good earth system but as I said the sooner the better for upgrade.
There are many TNS systems around with no Earth leakage protection at all...untill last update of regs I would have suggested as a cheaper alternative to changing the CU get an RCD directly in place of the VOELCB,
but as will not comply with 17th edition I doubt anyone will do that now,