How much voltage drop is normal?

[ccam108]

I moved in to a new (rented) flat a few months ago and the voltage drop when appliances are switched on is terrible. The lights noticeably dim whenever anything is turned on.

I measured the voltage at a socket in the bedroom with and without the shower on (not the most scientific I know, but I wasn't going to start taking the CU cover off for a quick check). I got 238V with the shower off and 228V with it on. The shower is 9kW.

Should I be contacting the DNO about this kind of drop or should I just go and buy some more CFLs so I don't notice it as much?

 

[westie101]

Well as the declared voltage is 230V +10 -6% what you have is well within those limits.

It would do no harm to talk to the DNO as you are seeing a significant dip which could be in part caused by poor connections around your consumer unit (are there any signs of overheating or smells of burning?)

If that area seems OK then it could be a network issue, or even that you are a long way to a substation or cable sizes are smaller than normal if it is an older network.

 

[ccam108]

Thanks for the reply.

I haven't noticed any signs overheating around the CU or meter, but will keep an eye on it.

I'm very near the town centre of a large town and the nearest substation (not necessarily the one supplying me of course) is around 200m away, so my money is on a small supply cable.

In which case, I take it the DNO won't really care?

 

[westie101]

In which case, I take it the DNO won't really care?

Possibly not, but it might be worth you reporting the problem to their fault line, it could indicate other network problems

 

[JohnW2]

If that area seems OK then it could be a network issue, or even that you are a long way to a substation or cable sizes are smaller than normal if it is an older network.

Interestingly, given that a Ze of at least 0.35Ω is regarded as acceptable for a TN-C-S supply, the correspondingly 'acceptable' L-N loop impedance would be at least as high as that (slightly higher, I suppose, given the effects of PME on Ze) - such that a voltage drop of at least 14V in response to a ~40A current might be 'expected' - whereas the OP is seeing only a 10V drop.

Kind Regards, John

 

[westie101]

Interestingly, given that a Ze of at least 0.35Ω is regarded as acceptable for a TN-C-S supply, the correspondingly 'acceptable' L-N loop impedance would be at least as high as that (slightly higher, I suppose, given the effects of PME on Ze) - such that a voltage drop of at least 14V in response to a ~40A current might be 'expected' - whereas the OP is seeing only a 10V drop.

In a town centre close to a substation I would expect the Ze to be a good bit lower than that anyway owing to probable interconnections on the LV network.
Depending on the length of the service cable a good proportion of that impedance could be there.

One of the reasons it is worth reporting is that we occasionally find a fuse contact overheating & burning, this can cause excessive voltage drops as well, so it is preventative to check on reports like this to prevent bigger problems in the future. We have one in a local town where a burnt contact means that we will probably have to replace a transformer & LV pillar owing to the lack of spares for 1960s built gear[/quote]

 

[JohnW2]

In a town centre close to a substation I would expect the Ze to be a good bit lower than that anyway owing to probable interconnections on the LV network. Depending on the length of the service cable a good proportion of that impedance could be there. One of the reasons it is worth reporting is that we occasionally find a fuse contact overheating & burning, this can cause excessive voltage drops as well, so it is preventative to check on reports like this to prevent bigger problems in the future.

Fair enough. However, it's just 'by chance' that the OP has noticed this. His figures compute to a Ze no higher than about 0.25Ω. Are you therefore suggesting that, for the reasons you state, DNOs would like electricians report to them all cases of Ze measurements apparently 'as high as' 0.25Ω on PME supplies?

Kind Regards, John

 

[westie101]

Er no,
We often measure low Ze but find connection problems. The issue often only appears under higher load conditions.
So measuring parameters can and cannot be an indication of problems.

 

[JohnW2]

Er no, We often measure low Ze but find connection problems. The issue often only appears under higher load conditions. So measuring parameters can and cannot be an indication of problems.

There seems to be some inconsistency here! As I see it, in any TN-C-S installation with a Ze of 0.25Ω (even with no 'connection problems'), one would expect to see at least 10V voltage drop when switching on a 40A load - so why do you suggest that the OP should report this (since he has 'noticed' this happening), but do not think that all the other installations with a Ze of 0.25Ω (in which consumers have not, yet, 'noticed' the voltage drop) should be reported. Can you perhaps clarify?

Kind Regards, John

 

[bernardgreen]

Is this a purpose built flat or a house converted into flats ?

If a converted house then maybe the network incomer is OK as regards supply impedance with most of the voltage drop occuring along a in-adequate sub-main from cut-out to the flat's coonsumer unit.

 

[JohnW2]

If a converted house then maybe the network incomer is OK as regards supply impedance with most of the voltage drop occuring along a in-adequate sub-main from cut-out to the flat's coonsumer unit.

That's possible. However, as I've said, if one assumes that it is a TN-C-S supply with a Ze of 0.25Ω (which many/most people would regard as 'OK'), then a fall in voltage of at least 10V (at the network incomer) is precisely what one would expect when switching on a ~40A load - so one doesn't necessarily need to postulate any downstream 'inadequacies' to explain what the OP is experiencing.

Kind Regards, John

 

[DetlefSchmitz]

John: It's exactly the point I was trying to make here:

http://www.diynot.com/forums/electrics/electric-shower-causes-light-to-dim.348871/page-2#2616996

I didn't get very far with it either.

 

[EFLImpudence]

I don't really follow the argument.

Everyone has a Ze (or Zn) of some value (~0.20 ?) but not everyone's lights dim when the shower is turned on.

 

[DetlefSchmitz]

Well we know the voltage is going to drop - Ohm's law. So maybe soe people's lighting is not so sensitive to voltage drop. (Different lamp technologies?)

Or maybe they only have showers in the daytime?

 

[westie101]

Never heard of customer service, the customer has a concern, it is not dangerous and is answered here, BUT he may still be concerned so needs to take it up with the appropriate people the DNO!

It doesn't cost him anything! But COULD save further problems in the future

Digging into the rights and wrongs of a measured or calculated value as evidence of an issue or not is foolish!

An observation of an issue that may have just started (we don't know the full history though) COULD be an indication of something else.
This is personal experience from exactly this occurring on a number of occasions, where the calculated & measured value of the Ze was well within limits but a 400A fuse had a burning contact!

Is what I'm saying in a book or legislation - NO, But if I was aware of similar in the patch I work I would investigate or get it investigated!

Other questions that would be asked: - load on the rest of the network, is the customer actually fed from a more distant substation with an open point at the one near him, is the normal substation back-fed for repair/maintenance/replacement, this can lead to flickering

We do not know, the DNO does!

How many times have we seen a similar question being answered with the advice to pay an electrician to come and check the CU?[/b]