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Prospective short circuit current and loop impedance are related and it does not really matter which is measured but what we look for is the worst reading which will be as near to centre of ring that we can measure.
There are two limiting factors one is the volt drop and the other is the current required to ensure the automatic disconnection device will disconnect within the prescribed time.
Before the advent of the RCD we would be more interested in the line – earth reading but today it’s the line – neutral reading which is more likely to be the limiting factor.
The gives us the first problem as although top of the range meters may measure PSCC line – neutral and ELI line – earth not all meters do this so often it means making leads to allow this to be done without having to resort to probes.
But the real question is what is the line – neutral limit? To trip a B32 MCB in 0.01 seconds then 1.44 ohms but volt drop is not so easy. We are given complex formula and rules like 20 amp in centre are 12 amp even spread so for volt drop we use 26 amp not 32 amp. But the loop impedance will work out at somewhere near 0.79 Ω assuming the supply at 0.35 Ω. The ELI likely will be around the 0.94 Ω mark.
Simple maths without correction factors 230/11.5 = 0.44 + supply resistance of 0.35 likely with a TN-C-S supply = 0.79 Ω. At 0.018 Ω per meter = 44 meters double it up and 88 meters use the correction factors 106 meters of cable which is fair enough.
But question is measure say 0.70 Ω and your not sure that’s dead centre and customer wants ring extending to 4 extra sockets. Up and down walls 3 meters each and between sockets another 10 meters. So you need to add 22 meters. This will likely add 0.198 Ω to the reading so 0.90 Ω total although you have measured hot and meters not really that accurate.
I have seen ELI measured with an EICR, but line – neutral I would think is rarely measured. So getting caught is remote. Will the extra volt drop cause a problem? unlikely. Running 4 fused spurs is a real pain and does not look professional.
Exceeding the 1.44 Ω could cause danger, but with a RCD this is removed, and I personally wonder what danger is there with excessive volt drop? To me the danger is some one some time in the future will find out, and use ones own paperwork to prove you knew it did not comply, and want you to correct FOC. So should it be written on the departures? At least that way you can say it was agreed at the time.
I tell the DIY guy before extending a ring final he should measure the loop impedance and work out if once his alterations are completed will it comply. But I really question how many professional electricians actually sit there and work out volt drop before quoting for the work? Once quote is done to then say can’t do it for that price because what ever is really too late. Seeing RCD missing is easy but how many even measure the loop impedance before starting the job?
So does it really matter? And if some one did measure the volt drop and ask for the problem to be solved what would be your answer? That’s assuming there is over 5% volt drop. Would you fix it? I have heard the answers so many times, I told your wife “xxx” and she said go ahead anyway. “Or I did tell you that before I started.” My answer would be of course then why is it not on the list of items not complying.
When I bought my house new in 1980 I was given no installation certificate and so there is no way I could prove what it was like when installed. Today there are reams of paper work so even 20 years on some smart owner could try it on. If the installation certificate shows it did not comply when installed we can hardly call it fair wear and tear. We are seeing where sex offenders are being taken to court some 40 years after the event. Could we be taken to court in 20 years time for work we have done today?
Personally I would say 10 year limit as after 10 years there should have been an EICR, but seems there is no such limit with sex offenders no one says why didn’t you say something when you became adult at 21.
So what is the opinion on volt drop limits? Does anyone really worry? And if one retires would insurance valid at time of the work still cover you? With my Vauxhall car sold to my daughter some 10 years latter she still had a new fuel tank fitted under a recall where Vauxhall found there was a design fault. We are here talking about a design fault so would there be any point where a customer could not ask for it to be fixed?
There are two limiting factors one is the volt drop and the other is the current required to ensure the automatic disconnection device will disconnect within the prescribed time.
Before the advent of the RCD we would be more interested in the line – earth reading but today it’s the line – neutral reading which is more likely to be the limiting factor.
The gives us the first problem as although top of the range meters may measure PSCC line – neutral and ELI line – earth not all meters do this so often it means making leads to allow this to be done without having to resort to probes.
But the real question is what is the line – neutral limit? To trip a B32 MCB in 0.01 seconds then 1.44 ohms but volt drop is not so easy. We are given complex formula and rules like 20 amp in centre are 12 amp even spread so for volt drop we use 26 amp not 32 amp. But the loop impedance will work out at somewhere near 0.79 Ω assuming the supply at 0.35 Ω. The ELI likely will be around the 0.94 Ω mark.
Simple maths without correction factors 230/11.5 = 0.44 + supply resistance of 0.35 likely with a TN-C-S supply = 0.79 Ω. At 0.018 Ω per meter = 44 meters double it up and 88 meters use the correction factors 106 meters of cable which is fair enough.
But question is measure say 0.70 Ω and your not sure that’s dead centre and customer wants ring extending to 4 extra sockets. Up and down walls 3 meters each and between sockets another 10 meters. So you need to add 22 meters. This will likely add 0.198 Ω to the reading so 0.90 Ω total although you have measured hot and meters not really that accurate.
I have seen ELI measured with an EICR, but line – neutral I would think is rarely measured. So getting caught is remote. Will the extra volt drop cause a problem? unlikely. Running 4 fused spurs is a real pain and does not look professional.
Exceeding the 1.44 Ω could cause danger, but with a RCD this is removed, and I personally wonder what danger is there with excessive volt drop? To me the danger is some one some time in the future will find out, and use ones own paperwork to prove you knew it did not comply, and want you to correct FOC. So should it be written on the departures? At least that way you can say it was agreed at the time.
I tell the DIY guy before extending a ring final he should measure the loop impedance and work out if once his alterations are completed will it comply. But I really question how many professional electricians actually sit there and work out volt drop before quoting for the work? Once quote is done to then say can’t do it for that price because what ever is really too late. Seeing RCD missing is easy but how many even measure the loop impedance before starting the job?
So does it really matter? And if some one did measure the volt drop and ask for the problem to be solved what would be your answer? That’s assuming there is over 5% volt drop. Would you fix it? I have heard the answers so many times, I told your wife “xxx” and she said go ahead anyway. “Or I did tell you that before I started.” My answer would be of course then why is it not on the list of items not complying.
When I bought my house new in 1980 I was given no installation certificate and so there is no way I could prove what it was like when installed. Today there are reams of paper work so even 20 years on some smart owner could try it on. If the installation certificate shows it did not comply when installed we can hardly call it fair wear and tear. We are seeing where sex offenders are being taken to court some 40 years after the event. Could we be taken to court in 20 years time for work we have done today?
Personally I would say 10 year limit as after 10 years there should have been an EICR, but seems there is no such limit with sex offenders no one says why didn’t you say something when you became adult at 21.
So what is the opinion on volt drop limits? Does anyone really worry? And if one retires would insurance valid at time of the work still cover you? With my Vauxhall car sold to my daughter some 10 years latter she still had a new fuel tank fitted under a recall where Vauxhall found there was a design fault. We are here talking about a design fault so would there be any point where a customer could not ask for it to be fixed?