Improved earth
- Thread starter sh
- Start date
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sh said:
the RCD trips because there is an imbalane between phase and neutral. i.e a fault. more earth rods will not stop and RCD trippin. is everything protected by a 30mA RCD? its possible that you have a few devices causing an earth leakage which is going over the 30mA limit and trippin the RCD
I currently have 30mA control on the incomer. This is what I am looking into at the moment. I think it is a combination of new kitchen appliances (oven, microwave combi, fridge/freezer and dishwasher) and older washing machine and tumble dryer.
Can you tell me the calculation for converting earth leakage mega ohms to RCD trip levels?
Thanks
Can you tell me the calculation for converting earth leakage mega ohms to RCD trip levels?
Thanks
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i would STRONGLY guess that changing that rcd will require pulling the service fuse which is not something we advise diyers to do (thouh there are certainly diyers who have done it)
any chance of a photo of the CU meter service cutout and any wiring between them?
any chance of a photo of the CU meter service cutout and any wiring between them?
I definately think it is a combination of appliances,I have just replaced the heating element in the w/m (only 3 years old and that seems to be behaving itself but the brand new oven in a newly fitted kithchen has a l-e and a n-e mega reading of 2.05.I think this in conjunction with additional standing earth leakage is tipping it over the 30mA.Any thoughts ?
sh,
Maybe my experience is relevent. I also have a TT system and recently upgraded my CU with RCDs and MCBs (from wire fuses and a huge old 500mA incomer RCD). I got a suitably employed person to 'pull the fuse' on the QT as a favour.
I agree that additional earth rods won't do anything for you. The whole point about TT CU installations is to provide protection with the higher earth loop resistance.
My installation now has a TIME DELAY 100mA 100A RCD as the first port of call in the CU. This feeds the lighting circuits, the fridge freezer socket, and the CH MCBs. The time delay function provides some protection from glitches when the CH or freezer cranks up.
This first RCD also feeds a second RCD which is 30mA 63A (non time delay). This feeds two rings for socket outlets.
I was fairly rigorous with testing, as far as I could with a Fluke DVM, checking loop continuity and isolation. The latter encompassing L-N-E isolation within a circuit, and also circuit to circuit isolation on each wire.
Note that all testing was carried out with bulbs & appliances disconnected (i.e. unplugged). One must have some confidence that the fixed wiring is doing what one expects. Otherwise there are too many variables in trying to isolate whether the fault is in the fixed wiring or in an appliance.
Even though I am a DIYer (but with an electronics engineering background), and I had to do a fair amount of reading around to be confident I understood enough to tackle this (and deal with unplanned problems like nuisance tripping), I have had absolutely no problems since the installation was done.
My final advice, if you want to solve this yourself, is to really understand every aspect of your installation, and which combination of lighting, outlets and/or appliances cause your problem.
HTH.
Maybe my experience is relevent. I also have a TT system and recently upgraded my CU with RCDs and MCBs (from wire fuses and a huge old 500mA incomer RCD). I got a suitably employed person to 'pull the fuse' on the QT as a favour.
I agree that additional earth rods won't do anything for you. The whole point about TT CU installations is to provide protection with the higher earth loop resistance.
My installation now has a TIME DELAY 100mA 100A RCD as the first port of call in the CU. This feeds the lighting circuits, the fridge freezer socket, and the CH MCBs. The time delay function provides some protection from glitches when the CH or freezer cranks up.
This first RCD also feeds a second RCD which is 30mA 63A (non time delay). This feeds two rings for socket outlets.
I was fairly rigorous with testing, as far as I could with a Fluke DVM, checking loop continuity and isolation. The latter encompassing L-N-E isolation within a circuit, and also circuit to circuit isolation on each wire.
Note that all testing was carried out with bulbs & appliances disconnected (i.e. unplugged). One must have some confidence that the fixed wiring is doing what one expects. Otherwise there are too many variables in trying to isolate whether the fault is in the fixed wiring or in an appliance.
Even though I am a DIYer (but with an electronics engineering background), and I had to do a fair amount of reading around to be confident I understood enough to tackle this (and deal with unplanned problems like nuisance tripping), I have had absolutely no problems since the installation was done.
My final advice, if you want to solve this yourself, is to really understand every aspect of your installation, and which combination of lighting, outlets and/or appliances cause your problem.
HTH.
You can't do that with a DVM - you need a proper insulation tester that zaps 500V down the cable..Careful_Bodger said:
I know he said that. I can't think, though, what meaningful form of L-N-E isolation there is to test for other than insulation resistance...IanDB said:
While I agree, a full 500 VDC insulation check is the proper way to do it, even a quick low voltage check for earth neutral leaks at a low voltage will find 'nails in the cable' type faults, and must be preferable to no attempt to test at all.
Similarly for earth continuity. I would always use a high test current reading of Zs as the final word, but a pre-emptive flick with a general purpose meter to see a near short reading is still of value, as it weeds out the really silly faults (like cut back cores in the light circuit) before getting the proper tester out (and possibly the nasty jolt it delivers to the unwary).
I think what we should be saying is that such multimeter tests are a good first step at picking out most faults, but should not be relied upon as the final word as to the condition of the circuit.
For example in extremis the old car battery plus lamp could make a good test of low impedance things with the pocket meter, as with a few amps flowing, the volts to ohms conversion is within the range of most digital multimeters, and the current flow is more realistic. (Personally while 200mA may be officially allowed, I still think its a bit light, and may report "OK" on earthing that is really a bit threadbare, and actually could blow out in practice.)
While I don't think we should exactly condone testing at voltages/currents below recommended, I think we should be careful not to give the impression that we'd prefer people not to test at all as an alternative.
regards M.
Similarly for earth continuity. I would always use a high test current reading of Zs as the final word, but a pre-emptive flick with a general purpose meter to see a near short reading is still of value, as it weeds out the really silly faults (like cut back cores in the light circuit) before getting the proper tester out (and possibly the nasty jolt it delivers to the unwary).
I think what we should be saying is that such multimeter tests are a good first step at picking out most faults, but should not be relied upon as the final word as to the condition of the circuit.
For example in extremis the old car battery plus lamp could make a good test of low impedance things with the pocket meter, as with a few amps flowing, the volts to ohms conversion is within the range of most digital multimeters, and the current flow is more realistic. (Personally while 200mA may be officially allowed, I still think its a bit light, and may report "OK" on earthing that is really a bit threadbare, and actually could blow out in practice.)
While I don't think we should exactly condone testing at voltages/currents below recommended, I think we should be careful not to give the impression that we'd prefer people not to test at all as an alternative.
regards M.
B
Big_Spark
Whilst some basic testing can be done with a DVM, nothing of any real importance will come of it.
Lets look at the facts.
You have a TT system with an Earth Rod.
You have an RCD Incomer rated at 100A 30mA
Your RCD keeps tripping.
You have new appliances and old appliances in your home, and you obviously have a computer.
Now lets look at the causes of the Nuisance tripping:
The RCD is simply buggered...
The Leakage on the Earth is such that at times it pushes sufficiently over the 30mA limit for in excess of 0.04 seconds and the RCD operates. This could be caused by a single fauly appliance,or more likely a combination of the leakage currencts from numerous devices. The most likely being Inductive loads and electronics. I would suggest that disconnect you main bond, then connect you DVM across this break, and set it to auto-range AC. Measure the voltage on the Earth with all the appliances OFF. Then go around and turn them on, one by one, and record the voltage that is registered on the DVM. Eventually you will get to the point where all the kit is on, or the RCD will have tripped. By noting the the voltages for each appliance, you are then in a position to calculate the leakage in mA on the Earth of your installation. You will need to determine the resistance of the Earth path from the appliance to the CCU, but then you can simply use Ohms law.
The easier way, if you have one, is simply use a clamp meter across the main Earth, whilst still connected, and then turn on the appliances, one at a time, and note the current on the Earth. You will either realise you don't have a leakage issue, but an RCD issue, or it will be a leakage issue that requires and different rating, and time delayed RCD. You even find that it is one appliance causing the problem by itself.
Lets look at the facts.
You have a TT system with an Earth Rod.
You have an RCD Incomer rated at 100A 30mA
Your RCD keeps tripping.
You have new appliances and old appliances in your home, and you obviously have a computer.
Now lets look at the causes of the Nuisance tripping:
The RCD is simply buggered...
The Leakage on the Earth is such that at times it pushes sufficiently over the 30mA limit for in excess of 0.04 seconds and the RCD operates. This could be caused by a single fauly appliance,or more likely a combination of the leakage currencts from numerous devices. The most likely being Inductive loads and electronics. I would suggest that disconnect you main bond, then connect you DVM across this break, and set it to auto-range AC. Measure the voltage on the Earth with all the appliances OFF. Then go around and turn them on, one by one, and record the voltage that is registered on the DVM. Eventually you will get to the point where all the kit is on, or the RCD will have tripped. By noting the the voltages for each appliance, you are then in a position to calculate the leakage in mA on the Earth of your installation. You will need to determine the resistance of the Earth path from the appliance to the CCU, but then you can simply use Ohms law.
The easier way, if you have one, is simply use a clamp meter across the main Earth, whilst still connected, and then turn on the appliances, one at a time, and note the current on the Earth. You will either realise you don't have a leakage issue, but an RCD issue, or it will be a leakage issue that requires and different rating, and time delayed RCD. You even find that it is one appliance causing the problem by itself.
One of many possibilities;
Cooker elements can take a few weeks of use to stop leaking. I found one recently in a house with an AGA which was stuffing their RCD. They normally used the AGA 3 seasons (cooking & heating) and switched it off in the summer, using the Electric one for cooking then. Never a problem before as used in summer. This year loads of rellies coming to Christmas bash so the electric oven was used to prepare, RCD went every time the knob was turned up. Luckily they'd decided to cook a sunday lunch to give it an initial run.
Solved by doing a full morning of cooker on with the RCD incomer swapped out. Note they were all at work & school so no one else there. This is one of those 'don't try this at home folks' scenarios unless you can guarantee your own and everyone elses' safety. It could be a new heating element needs to shed some moisture. You could try a very low heat for a long period if you suspect the cooker. Switch all off and one by one switch on the next until you get to the culprit. If it's intermittent then look out for the cooker & fridge / freezer etc which kick in intermittently.
Cooker elements can take a few weeks of use to stop leaking. I found one recently in a house with an AGA which was stuffing their RCD. They normally used the AGA 3 seasons (cooking & heating) and switched it off in the summer, using the Electric one for cooking then. Never a problem before as used in summer. This year loads of rellies coming to Christmas bash so the electric oven was used to prepare, RCD went every time the knob was turned up. Luckily they'd decided to cook a sunday lunch to give it an initial run.
Solved by doing a full morning of cooker on with the RCD incomer swapped out. Note they were all at work & school so no one else there. This is one of those 'don't try this at home folks' scenarios unless you can guarantee your own and everyone elses' safety. It could be a new heating element needs to shed some moisture. You could try a very low heat for a long period if you suspect the cooker. Switch all off and one by one switch on the next until you get to the culprit. If it's intermittent then look out for the cooker & fridge / freezer etc which kick in intermittently.
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