I'm replacing plastic with metal plates. The existing earths run to the back box first and then run a fly to the back of the plate. Is it ok to leave this arrangement for metal plates?
Earth to back box first
- Thread starter Squiddley57
- Start date
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Yes.
Yes that is fine, are the new switches metallic? As they will require connecting to earth for safety reasons, the one problem is do you actually know you have CPC/earth continuity?
Just because they are there does not automatically mean they have a path to earth.
Just because they are there does not automatically mean they have a path to earth.
Once I've done the job (one of many) i guess that I shall have to get an electrician in to do a full check. There's no other way?
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Yes. Buy a simple multimeter and, with the power off, test continuity from each switch point back to the main earthing terminal of the installation.
I've got a multimeter but I've not used it for testing continuity. Can you point me towards explanation/guidance?
Well, the instruction book will tell you. But see this bit on YouTube. Look for teh bit on continuity (starts at about 2:30).
You'll need one probe on the earth on the box and the other all the way back to the main earth terminal at teh consumer unit/intake, so you'll need a long length of wire…..
You'll need one probe on the earth on the box and the other all the way back to the main earth terminal at teh consumer unit/intake, so you'll need a long length of wire…..
Ideally
Set meter to low ohms.
Place one probe on the part or conductor you want to test and the other on the main earth terminal near the consumer unit.
You will obviously require a long lead.
Before testing measure the resistance of the leads and the long lead, then deduct this form the test measurements.
Result should be less than one ohm.
Easier but not conclusive
Place the second lead on a known earth: screw on nearby socket (with metal back box of course), appliance carcass (hob, oven) or gas pipe etc.
Result should be less than one ohm
Set meter to low ohms.
Place one probe on the part or conductor you want to test and the other on the main earth terminal near the consumer unit.
You will obviously require a long lead.
Before testing measure the resistance of the leads and the long lead, then deduct this form the test measurements.
Result should be less than one ohm.
Easier but not conclusive
Place the second lead on a known earth: screw on nearby socket (with metal back box of course), appliance carcass (hob, oven) or gas pipe etc.
Result should be less than one ohm
Blimey I shall need a long lead. can I use a roll of earth lead, clamp it to the earth at the consumer unit, and use that?
sure. As described above, measure the resistance of the reel and then deduct that from the total that you measure.
By setting the multimeter to AC voltage you can test across the com of the switch and earth terminal, if you get a reading of about 240V, then that would suggest there is a path to earth, this is a live test so make provision to test safely.
You can also do the the r2 test between earth/CPC at switches and the earthing busbar at the CU/Board.
or you could test a earth of nearby socket but if you to do this you would also need to confirm that the socket had a path to earth.
Other method is to identify the CPC/earth and live line at the CU/Board.
Isolate the protective device, test the top of the device is dead, then remove the liv/line conductor and terminate this into the earth busbar, then go to each switch testing at low ohms across live/line and CPC.
The further you go up the circuit the greater the measurement will read.
The results will vary depending on the length of the circuit, it's normally around the 1 ohm mark, sometimes a little higher, but providing the readin is less than 6.0 ohms, that should satisfy most 5A-6A devices disconnection times.
You can also do the the r2 test between earth/CPC at switches and the earthing busbar at the CU/Board.
or you could test a earth of nearby socket but if you to do this you would also need to confirm that the socket had a path to earth.
Other method is to identify the CPC/earth and live line at the CU/Board.
Isolate the protective device, test the top of the device is dead, then remove the liv/line conductor and terminate this into the earth busbar, then go to each switch testing at low ohms across live/line and CPC.
The further you go up the circuit the greater the measurement will read.
The results will vary depending on the length of the circuit, it's normally around the 1 ohm mark, sometimes a little higher, but providing the readin is less than 6.0 ohms, that should satisfy most 5A-6A devices disconnection times.
By setting the multimeter to AC voltage you can test across the com of the switch and earth terminal, if you get a reading of about 240V, then that would suggest there is a path to earth, this is a live test so make provision to test safely.
You can also do the the r2 test between earth/CPC at switches and the earthing busbar at the CU/Board.
or you could test a earth of nearby socket but if you to do this you would also need to confirm that the socket had a path to earth.
Other method is to identify the CPC/earth and live line at the CU/Board.
Isolate the protective device, test the top of the device is dead, then remove the live/line conductor and terminate this into the earth busbar, then go to each switch testing at low ohms across live/line and CPC.
The further you go up the circuit the greater the measurement will read.
The results will vary depending on the length of the circuit, it's normally around the 1 ohm mark, sometimes a little higher, but providing the reading is less than 6.0 ohms, that should satisfy most 5A-6A devices disconnection times.
You can also do the the r2 test between earth/CPC at switches and the earthing busbar at the CU/Board.
or you could test a earth of nearby socket but if you to do this you would also need to confirm that the socket had a path to earth.
Other method is to identify the CPC/earth and live line at the CU/Board.
Isolate the protective device, test the top of the device is dead, then remove the live/line conductor and terminate this into the earth busbar, then go to each switch testing at low ohms across live/line and CPC.
The further you go up the circuit the greater the measurement will read.
The results will vary depending on the length of the circuit, it's normally around the 1 ohm mark, sometimes a little higher, but providing the reading is less than 6.0 ohms, that should satisfy most 5A-6A devices disconnection times.
Could I test by running a lead from the earth on the nearest 13a socket to the light earth and vice versa?
My preference is to connect the CPC to the socket plate first, then a flylead to the backbox.
A little bit easier, as fitting 2 cables into the backbox terminal can be pretty fiddly. And also because I use stranded (solid strands) earth cable, and I think you should not mix this in a terminal with a solid core. Although this may be just for flex. type cables.
A little bit easier, as fitting 2 cables into the backbox terminal can be pretty fiddly. And also because I use stranded (solid strands) earth cable, and I think you should not mix this in a terminal with a solid core. Although this may be just for flex. type cables.
You could, but it won't provide the results you were looking for.
If it was open circuit, that could mean the earth in the socket was faulty, rather than the light. Or perhaps both were defective.
If it showed continuity, that could mean all is well.
Or perhaps the socket earth connection was only there due to some other connection elsewhere, such as via an immersion heater or boiler.
Or maybe the two CPCs (earths) for each circuit were connected together, but neither of them were actually connected to earth.
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