Electric shower causes light to dim

[westie101]

440W being dissipated somewhere

If in the vicinity of the cut-out/meter/CU there would be obvious signs of that heat dissipation, the OP has said he's checked the area and there are no signs of problems.

But in any case all he needs to do is run the shower for a while and check for any heat in the area

However if he is still concerned a call to the DNO will get someone out who is authorised to check it all for no charge.

 

[RF Lighting]

I see this a lot with large electric showers. I would think it is the exception rather than the rule to see no volts drop at all.

 

[ebee]

Yep Correct.
It depends upon how much is dropped and where.
If we measure at CU terminals with no load then on shower load then we get some idea about our supply (ideally we should check at the suppliers terminals before the meter but that would involve naughty seal removal so I`m not going there).
Then we measure at shower unit and can deduce the volt drop in the cable from the CU to the shower.

From what the OP has said (he checked at his sockets) suggest the origin of his supply or somewhere thereabouts is dropping - this might suggest his incoming supply is weak or the tails from cutout to meter are weak or his CU tails or a loose connection somewhere in there.

If his supply dipped initially then almost immediately recovered a lot but not completely then this would be OK

 

[Sinful]

Yep Correct.
It depends upon how much is dropped and where.
If we measure at CU terminals with no load then on shower load then we get some idea about our supply (ideally we should check at the suppliers terminals before the meter but that would involve naughty seal removal so I`m not going there).
Then we measure at shower unit and can deduce the volt drop in the cable from the CU to the shower.

From what the OP has said (he checked at his sockets) suggest the origin of his supply or somewhere thereabouts is dropping - this might suggest his incoming supply is weak or the tails from cutout to meter are weak or his CU tails or a loose connection somewhere in there.

If his supply dipped initially then almost immediately recovered a lot but not completely then this would be OK

Hello,

I took some more measurements today, from the CU main switch, I was still seeing a 9v drop when the shower was on load.

I also took a L-N loop measurement which was 0.18, it's a TNS system, not sure what's typical. But would that not account for the volt drop on the supply?

I still haven't done any seal breaking and had a look at the tail connections (although I'm tempted) but what I did do is set up a voltage recorder and gently move the tails to see if there was any fluctuation, there was not.

I'd like to swap the tails for some 25mm's, then I think I can forget about it.

I'll do the test at the shower isolator tomorrow and see what voltage drop is occurring there.

 

[ebee]

Can you just clarify "Volt Drop" please?

Is this the volt drop from one end of the cable to the other at (substantially) the same time on load

or

is the the volt drop difference at your CU terminals between shower ON load and Off load ?

And, if the latter, did you wait a few seconds to see if the voltage crept back again with load still running?

 

[Sinful]

Can you just clarify "Volt Drop" please?

Is this the volt drop from one end of the cable to the other at (substantially) the same time on load

or

is the the volt drop difference at your CU terminals between shower ON load and Off load ?

The latter

 

[ebee]

It all indicates that the supply is dipping on load then.

If it does not creep back up quite quickly whilst running then I`d be inclined to ask the DNO if it sounds about right for you supply.

I would expect some dip but not 10 volts (if we knew what the terminal at the cutout was we could narrow it down to being completely your suppliers volt drop or not)

 

[westie101]

From quite a few years of dealing with exactly this type of issue, if there are no signs of problem then, yes, it is not unusual.

I get about 6 volts dip with a 9.5kW shower.
500kVa transformer, 120m 4c .3 Aluminium cable, 100m 3c 185 mm2 Al, 140m 3c 95 Al cable and a 10m 25mm2 copper service.

Customers further from substations or on longer lengths of smaller cable or long services will see worse reductions, even the size of & loading on the transformer can make a difference.

It is within limits and will not be causing any other problems to equipment in the house.

 

[DetlefSchmitz]

Can we put the question like this? What is the range of typical mains input impedances expected in a UK domestic supply?

Well no experts came forward but 0.25 ohms is not untypical according to the internet. Given that on PME, Ze is allowed to be 0.8 ohm, and the earth and neutral are the same wire, I don't see why there is any surprise at 44A causing a 9V drop.

The OP measures .18 ohms as close as he can get to the source, confirming the situation within about a volt.

I'm just surprised everyone seems to be assuming 0 ohms impedance from the main supply.

Anyway I see Westie has replied while I was typing - it would be good if he has a figure for the L-N impedance range of values, as accepted by the DNOs.

 

[westie101]

The only ones we look at are Ze, which as you say are acceptable up to .8 ohm.

The only time we would really look at it would be the design stage for new networks to ensure volts remain within limits (which they do in this case).