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Volts drop when 2.5kW (ish) load switched on

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This is just a starter because I haven't done any measurements yet (I was a guest there) and what I can do will be restricted as this has never been thought to be a problem by the owner who is a relative, but I'm more concerned than he is by what I've seen.
Background: modern hugely extended/rebuilt house (probably first built in 80's but rebuilt about 10 years ago).
100A main fuse, two large Wylex RCD split CU's. It looks like the cut-out has probably been relocated as it's now close to the CU's, but not near an outside wall now, which is where it probably was before the house was effectively rebuilt. (I need to find out if it was originally an outside wall, will probe the rebuild history gently next time I'm there)
The tails, 25mm are fairly short (45-60cm)
There are lots of separate circuits, up and down rings, kitchen ring, various radials, various lighting circuits etc.
The lighting in the kitchen (and most of the rest of the downstairs) is LED GU10's but previously it was halogens. (35 or 50W not sure which, but 6A MCB on a separate circuit going back to the CU)
When the kettle is switched on you can see the lights dim, only slightly but certainly enough to notice. I saw this same thing with the halogens before they were changed to LED lamps (and with the previous kettle).
I haven't had the opportunity to do any voltage measurements yet but I suspect that any similar load being switched on in other parts of the house will likely have the same effect.
The lighting load is probably no more than 100 - 200W on that circuit now. (I didn't count but certainly no more than 20 GU10 at 6 or 7 W max)
My gut feeling is that the voltage drop must be quite large to notice the lights dim even if it's only slight, and worryingly with such a relatively small load increase. Obviously I need to do some tests, at the minimum measure the voltage drop and the current as the kettle is switched on (and when the oven or any other highish load appliance is switched).
Although very unlikely to be relevant, the standing (quiescent) consumption of the house is probably fairly high by modern eco-standards (I'm guessing well over 1kW) as lots of electronics is permanently powered, Alarms, cameras, Audio, satellite etc.

Thoughts anyone?
It sounds like something is high resistance somewhere common to both circuits - a joint in the cutout or somewhere along the tails perhaps. Are there Henley blocks feeding the two CUs? Could be a place to start. It would be good to find out if the circuits on both CUs are affected or just one.

It would be interesting to see what measurement you get for the volt drop when the kettle is switched on.

It sounds like quite a large installation hanging off what is presumably a single phase supply - is there a chance it could be being overloaded?
I've witnessed this happening before, in old properties with tungsten lamps. Surprised to hear it happens with LED's though... I thought they would give off the same brightness on voltage fluctuations and would flicker rather than dim.

You don't say how "big" this house is.... has it been extended from a 2 bed to 4 bed for example. It could be perfectly normal.

Is the house away up a lane on its own or in the middle of a street?
You can get funny effects if the house is on the end of a long suppliers cable.

1kW seems a little high for a standing load. Never actually checked mine, but I would reckon 100 to 200W for just electronics, fridge etc.
The most likely and first thing you should check is loose terminations within the switchgear..

Starting with the incoming tails and then the in and out of each bank of incomers/RCDs..
With the power off physically give them a good bit of movement, very common for the cables to not be actually tight in the terminals but rather in behind the terminal block
Thanks for all the replies, I'll post back when I've had the chance to measure the voltage (may be a while). It's been like that for years, I remember commenting on it when the halogens were there.
(The standing current is high (I guessed it but I will measure) because they have a lot of entertainment equipment on permanently - great in Winter at least the heat is not wasted, not so good in summer!)
I agree with the comment about the LED lamps, even with a 3% decrease in voltage I think you'd hardly notice a brightness change which is why I'm concerned.
To put some guesses in, (premature I know), lets say 20A (= kettle +standing+lights etc) is causing a volts drop of 3% (7.4V) = approx 0.37Ω, that resistance would be generating 148W ( I^2 R).
Yes, there are Henleys feeding the CU's, the laying on of hands on them gently felt no warmth (that was as far as I dared go).
I'll try to find out where the supplying substation is relative to the house, it's in a cul-de-sac but supply could be from the rear so may not be too far.
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I agree to check all common connections at the CU and upstream. So often over the years of the AC cycling can big cables become loose in their terminations - that’s why we all shout about double terminal screws all the time - probably wouldn’t have needed AMD3 of the 17th if everything was built to not loosen on its own!
I’ll put my wallet away now - I’ve had my tuppence worth!
The quiescent load, size of installation etc is more or less irrelevant to tracing the fault. With a supply of resistance R, when the load of one extra kettle consuming I amps is added, the voltage drop will increase by V volts, irrespective of any other circumstances. (This is a good approximation in this scenario although not 100% accurate)

One thing to note if the lighting that is dipping is on a dimmer; sometimes dimmers react to voltage changes with excessive variation in output. I.e. the dip effect is exaggerated beyond what you would observe if the same change in voltage was applied directly to the lamps.
Lucien I agree, but the quiescent isn't irrelevant to the amount of heat generated in that R (if there is an R somewhere). Good point about a dimmer though I don't remember seeing one...I'll check.
This thread hasn't been replied to for 14 days, so replying to this one may not get a response. Post a new thread instead.

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