im working on an old property which has radials running everything.it looks like TT. there is a 6mm earth coming from the old wylex cu but i have no idea where it goes. i did a Ze on it and it came out at around 70 ohms. when i plugged in my megger at the sockets, the earth loop readings are coming out at about 1.7 ohms. how can that be? surely they should be up around 80 ohms shouldn't they? the supply is overhead and the cu is situated on the landing of the house.
earthing
- Thread starter sijaysee
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interesting.......have you come across this before? there is a ELCB fitted and a 6mm earth which is why i was sure it was TT, but like i said before ,the readings at the sockets are so low and i have not been able to find the other end of the earth cable. if it is sharing a TNS or PME , where would the join be? the house is at the end of 3. the house at the other end has what looks like a mains cable which comes down the wall and dissapears into ground through a rainwater gulley!
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Yeah. I've found TT services before with NO main earth connection and a Zs of less than 1 Ohm. It's down to a main earth connection to the water pipe (which has been disallowed since 1966) and/or gas pipe (which has NEVER been allowed).
In your case, there seems to be a rod (of value 70 Ohm), but the connection to the pipework is bringing the reading down.
If it were me, I know what I would try, but I can't suggest it here!
But anyhow, as long as you have an independent connection to earth (your Ze was done by disconnecting the main earthing conductor and connecting your test meter to it directly, wasn't it?), that is under 200 Ohms (or 100 if your are of an NIC bent), then you're OK.
Just make sure your MEB's are bonded with min. 6mm2 and you have a split-load board with 100mA (min) TD S type RCD as an incomer, and 30mA on the split side. Oh, and that you use table 10C in the OSG to determine that the size of conductor back to the rod is OK.
In your case, there seems to be a rod (of value 70 Ohm), but the connection to the pipework is bringing the reading down.
If it were me, I know what I would try, but I can't suggest it here!
But anyhow, as long as you have an independent connection to earth (your Ze was done by disconnecting the main earthing conductor and connecting your test meter to it directly, wasn't it?), that is under 200 Ohms (or 100 if your are of an NIC bent), then you're OK.
Just make sure your MEB's are bonded with min. 6mm2 and you have a split-load board with 100mA (min) TD S type RCD as an incomer, and 30mA on the split side. Oh, and that you use table 10C in the OSG to determine that the size of conductor back to the rod is OK.
yea the Ze was done by disconnecting the earth from the consumer unit. that 6mm was the only earth though as i said , and it didnt go to the water main . dont know where it goes! the weird thing is though that i tested it again this morning and my loop readings were about 50 ohms. so i dont know if my tester was playing up or what! anyway i havnt touched the original installtion , i've just added a 2 way consumer unit with a 30 mA RCD to run my new kitchen ring and cooker radial. there was no water earth or gas earth so i've done those too. my Ze now is just under 50 ohms. am i right in saying that those old ELCB ' s dont comply with 16th edition?
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