High Zs

[Spark123]

1.43 ohms is just within the limit for a B32 MCB so can't see problem.
230/1.43 = 160.6 amps for B type divide by 5 = 32.2 so OK.

where are you getting 5 from i dont understand what is happening someone explain?

thanks

Did you actually read what I wrote here about conductor temperature at all?
A type B MCB trips (magnetically) in 3 to 5x its rated current. We use the higher of the two figures to calculate efli to ensure it operates.

 

[ericmark]

When we supply any electrical item we have to look at a number of different factors.

1) If we get a short to earth will it trip within the allowed time. Normally around 0.04 seconds to 0.1 seconds. This may be protected with a fuse, MCB, or RCD and with the latter then amount of current that needs to flow is very small.
2) If the line and neutral short circuit will it trip within the required time. Because in this case we are not so much protecting against shock the required time can be higher for example 5 seconds. This may help with a fuse but not so much with a MCB.
3) If under the design load the voltage will drop more than permitted level for power 5% and lighting 3%.

From the figures given the first one will pass using a RCD. But the second is not so cut and dried. If we take the 1.43 ohms and subtract the 0.58 ohms that is resistance using twin and earth where earth is likely 1.5mm and line is 2.5mm so we need to correct as neutral will be 2.5mm so resistance will be less. So line neutral impedance is more like 1.27 ohms.

This figure needs further correcting if the reading were taken hot but I will assume they were taken cold and as a result it would trip a B32 MCB on the magnet part of the device.

Here again we have to be careful so if it is under the current required for the magnet part then we need to calculate on the thermal part. On figure 3.4 showing tripping times it shows once the current drops below that needed to work magnet part of trip it will take over 10 seconds to work thermal part.

The last is volt drop here we are only concerned with volt drop after main fuse and again we need to correct as 2.5mm rather than 1.5mm so 0.688793 ohms but instead of the full 32 amp we are allowed to assume that 20 amp is drawn in centre and the remaining 12 is even spread so we work on 26 amp only. 26 x 0.688793 = 17.9 volts and we are only permitted 11.5 volts. As we reduce the supply current we still have to consider 20 amp draw at centre so 25 amp MCB would be calculated as 22.5 amp until we hit the 20 amp so 16 amp x 0.688793 = 11.02 volts and would be the largest MCB which could be used and comply with the regulations.

Since the 5% has only in last year been permitted and it was until the 17th Edition set at 4% the amount now has increased to 106.4904 meters from 85.19228 meters so we can now use an extra 21.29812 meters which is quite a lot of extra cable.

Either the ring main was well over size in the first place or it must be a hell of an extension.

The problem is of course just as I can sit at my PC and stick the reading into my excel program and churn out the results so can anyone else. And although there may be a margin of error both in the reading and in the ambient temperature of the cable when the reading were made it has not just missed the mark but has missed it by a good margin.

So how to get around the problem? I would look at returning original ring main to how it was. And running a new circuit to the extension. I have in the past reduced a MCB to 25 amp but to reduce the ring main to 16 amp it is likely to cause problems.

These mistakes are easy made when electricians are not provided with earth loop impedance testers. And it is quite common to employ one guy to do all inspection and testing and not to issue test kit to the installers. In an ideal world of course the guy testing should have tested the house before as well as after but we don’t live in an ideal world.

So we know a mistake has been made. No good harping on about it. What is wanted is methods one can use to correct it. So lets help the guy and any suggestions on how to correct?

 

[securespark]

eric- it is a big circuit: 1 RF for the whole house, including kitchen.

 

[Spark123]

When we supply any electrical item we have to look at a number of different factors.

1) If we get a short to earth will it trip within the allowed time. Normally around 0.04 seconds to 0.1 seconds. This may be protected with a fuse, MCB, or RCD and with the latter then amount of current that needs to flow is very small.
2) If the line and neutral short circuit will it trip within the required time. Because in this case we are not so much protecting against shock the required time can be higher for example 5 seconds. This may help with a fuse but not so much with a MCB.
3) If under the design load the voltage will drop more than permitted level for power 5% and lighting 3%.

From the figures given the first one will pass using a RCD. But the second is not so cut and dried. If we take the 1.43 ohms and subtract the 0.58 ohms that is resistance using twin and earth where earth is likely 1.5mm and line is 2.5mm so we need to correct as neutral will be 2.5mm so resistance will be less. So line neutral impedance is more like 1.27 ohms.

It is impossible to say as we don't know the supply type, if it is TN-S then the L-E impedance may bear no resemblance to the L-N impedance.

This figure needs further correcting if the reading were taken hot but I will assume they were taken cold and as a result it would trip a B32 MCB on the magnet part of the device.

I'd have thought it would be the opposite way around - if the readings are taken with the circuit cold then we need to know that it will still be within limits when it is at its maximum working temperature as copper has a positive temperature coefficient.

Here again we have to be careful so if it is under the current required for the magnet part then we need to calculate on the thermal part. On figure 3.4 showing tripping times it shows once the current drops below that needed to work magnet part of trip it will take over 10 seconds to work thermal part.

We don't use the thermal part to calculate the maximum efli on an MCB.
If it is providing fault current protection it has to operate on the magnetic part.

So we know a mistake has been made. No good harping on about it. What is wanted is methods one can use to correct it. So lets help the guy and any suggestions on how to correct?

Stick it on a 30mA RCD so the max permitted efli becomes 1666 ohms?

 

[ericmark]

My house was built the same so I ran a second ring for extension from which I have also fed a few other sockets.

My son did a refurbishment on an old mill which was being turned into a house and we argued as how to get around the problem of the property being so big. I wanted to run a 10mm twin and earth to far end of house into a cooker outlet than start and finish second ring into that.

My son rightly said that someone seeing a 10mm cable in a consumer unit may be temped to put a MCB of wrong size and he didn't thing you should change cable sizes without using a protection device where the cable changed size.

The builder did not want us to fit a sub-board.

All of course valid points and really speaking there is no right and wrong answer.

Which is why I asked for suggestions. We can all with hind sight say he got it wrong but that does not help what we want is ideas on how to get around the problem.

Splitting an existing ring main into two is not easy. But maybe there is room to supply some part of house with SWA and run it out side rather than through the house.

Or maybe when he looks at it he realises there is no way there is 106 meters used and there must be a fault some where.

I personally do not think he has a problem with tripping a B32 MCB and if that is the case then is the extra volt drop really a problem?

I do take on board what has been said about temperature of cable and compensating but I also wonder how often are tests done with hot cables? If the cable was supplying de-humidifiers just before the test it could be hot but that is not so likely.

I would want to use a low ohm meter and compare the reading when one is well within parameters using an ELI tester is OK but when on the limit I would want to double check the reading.

Anyway I wish him look which ever way he goes, and am glad it's not me who has to find way out.

 

[Spark123]

I have never known a test with the cables hot.
The fact is, that a cable can have a fault on it wether or not it is hot or cold. If it has a fault on it when it is cold then your EFLI will be low enough to trip a B32 within 0.1s
If the cable is loaded with dehumidifiers so the cable warms up to 70ºC and then someone sticks a chisel through the cable at the furthest most point then can you guarentee that the efli is low enough to disconnect a B32 MCB?
Nothing wrong with using 10mm to overcome volt drop, I had to install some 6mm for a 20A circuit not too long ago for that purpose.

 

[ericmark]

Sorry I was writing as Spark123 posted.
I took it for granted he would use a RCD as it supplies sockets under 20 amp. As a result I was not looking at Earth v Line impedance but Neutral v Line impedance.

I would point out the note on page 50. "NOTE 2:* The resistance of the installation earth electrode should be as low as practicable. A value
exceeding 200 ohms may not be stable."

Although it is referring to earth electrode I would consider anywhere where the ELI is over 200 ohms that something was very wrong.

I did try substituting 20 for 30 with formula given on page 258 and it allows another 2 meters of cable to be used. And changing to 70 to 90 allows another 4 meters to be used so not really of much consideration but as I have said the reading will be Earth to Line which will be protected by the RCD so only neutral to line which is of interest.

If he has no RCD it is not complying with BS7671 anyway so why worry about breaking one rule if your not worried about breaking another.

Which is why I did not dwell too much on tripping figures for B32 when it was so well out of speck with volt drop.

And he does state the fuse board was updated to comply with 17th Edition so I think it is reasonable to expect it does have RCD's fitted.

 

[Spark123]

To me, volt drop is more of a convenience thing where as max EFLI is to do with safety. It is a nightmare to calculate VD on a RFC anyway as not all current is drawn at the mid point and there are parallel paths.
Even though the max permitted value is 1666ohms I'm not implying that anyone should ever allow this, unless it is deliberately been introduced that way
Depending on cable lengths and sizes any large reading should set alarm bells ringing! I wouldn't entertain anywhere near 200 ohms on a socket circuit where it should be less than one ohm.

 

[ban-all-sheds]

All of course valid points

Apart from the last one:

The builder did not want us to fit a sub-board.

Not his call - he is unqualified and has engaged the services of a professional to carry out tasks which are beyond his competence.

I personally do not think he has a problem with tripping a B32 MCB

You're entitled to your personal opinion, but it would be unfortunate though if he found out the hard way that the maxima in BS 7671 were accurate though...

I do take on board what has been said about temperature of cable and compensating but I also wonder how often are tests done with hot cables?

They aren't, which is why there's the ROT for measured impedance.

 

[ban-all-sheds]

Even though the max permitted value is 1666ohms I'm not implying that anyone should ever allow this.

I didn't think that you were allowed to use RCDs to get around over-limit EFLI values on TN systems...

 

[Spark123]

It was spelt out that way in the 16th edn regs that they preferred the OCPD to operate, however in the 17th afaict 411.4.9 allows us to use the max efli figures from table 41.5 for a circuit not exceeding 32A protected by a non time delayed RCD to BS 61008-1 and BS61009-1.

 

[flameport]

in the 17th afaict 411.4.9 allows us to use the max efli figures from table 41.5 for a circuit not exceeding 32A

That is correct. I don't personally consider that regulation to be acceptable, as this is leading to the 'just shove an RCD on it' mentality, rather than designing the circuit properly.

 

[ban-all-sheds]

The IEE equivalent of Hill Descent Control...

 

[securespark]

I would find that regulation useful for situations where under a maintenance contract, you find a high Zs but the customer will not pay for upgrade or rectification work.

The other thing you can do with sockets with a slightly high Zs is to fit a 20A CPD.

eric- as to your RF problem, I have two comments:

I agree with Ban: it is the spark's call whether to fit a sub-main - stuff the builder!

Second, what about a 4mm² RF circuit?

 

[ericmark]

All valid points. My problem with builder long gone just pointing out different methods to cure problem.
Thanks through for suggestion on using 4mm.
But other than that all negative suggestions. Never mind what he has done wrong how can he correct it?
I agree can't take chance of neutral v line impedance but as yet only have calculated value.

To me with the use of RCD's on all circuits the neutral v line impedance is far more important than earth v line impedance but old habits seem to die hard.

Until I see a neutral v line impedance posted I am going to take the attitude the guy has given up. Not suppressing with so many negative posts and will give up on this one.