Water heater causes lights to dim

[ianbrookes]

please see the attached diagram.

This is a weird one and I am thinking that it is a supply issue. Has anyone seen this before.

The water heater is an instantaneous heater for a basin at 9.5kw and is in the garage. When it is turned on the lights in the house dim for the duration of the water heater being on. (diagram helps to see how garage and house are connected).

I measured the voltage drop as being 242v to 230v when the water heater is turned on. So not enough current ? or something else on the suppliers side ?

I have checked terminals etc and all cables are tight.

The house / garage are semi rural and it is the last house down the lane.

 

[oxocube]

Is the heater new or did it work ok before? Unwanted dmming is always due to high resistance, i.e. loose connections or inadequate supply cables.

 

[malcolmsanford]

You maybe having a volt drop problem on the phase that is coming into the house.

As the property is the last on in the lane, depending what is also on the phase could be giving you a voltage drop at origin, when the 9.5Kw load is applied.

Also that 6mm is pretty near it's mark for a 9.5Kw load, as i'm assuming it is SWA buried. How far is the run?

 

[ianbrookes]

thanks for the replies.

no loose connections.

the SWA is burried.

the point is that the kitchen lights dim - they are on a house CU circuit and the water heater is on a garage CU circuit.

 

[ianbrookes]

the SWA cable run is 29 meters. I took a voltage measurement at 242v in the garage, and 230v in the garage when the water heater is turned on.

 

[ianbrookes]

6mm SWA run of 29 meters. I measured the voltage at 242v in the garage dropping to 230v when the water heater is switched on. The lights dim in the house as well as the garage (and they stay dim until the WH is turned off). The house is on it's own CU fed via 6mtrs of 16mm from a switch fuse next to the meter.

 

[telectrix]

if you've still got 230V in the house when the water heater is on, then i'd look for a fault on the lighting.

 

[ianbrookes]

I just spoke to my Part P scheme technical help and the advice was that I have a supply issue and should contact my DNO (who are likely to not be interested).
I guess I will have to ask them to turn the wick up - or whatever the technical term is...

 

[Knobhead]

All I’ll say is more fool them for having a system like that installed on an O/H supply!

But when all is said and done the voltage is within tolerance.

 

[malcolmsanford]

post #3 me-thinks

 

[telectrix]

or... do like 1 of my client's did when faced with a supply of 207V... get 'em install 3 phase supply. only cost him £15,000

 

[ianbrookes]

O/H!!! don't know what you mean. if overhead then you should read the post and see the diagram... this is a TN-S not a TT.

 

[HandySparks]

Try doing the voltage change measurements at the switch fuse. This is the last point where the water heater and lights share a supply. It's only the shared cabling that matters as far as one load influencing another is concerned. You should find that the change in voltage when the heater is turned on is much smaller there, but still enough to be noticed. The human eye is very sensitive to small changes in light level.

It would be interesting to know just how small a change in voltage is regarded as a problem by the householder.

 

[Rockingit]

Table 4D4A refers:
Max rating of 6mm/3 SWA buried is 38A. 9.5kW @ 240V = 39A, 9.5kW @ 230V = 41A (depends on what voltage the manufacturer is using to claim 9.5kW).
Voltage drop is 6.9mV/A/m = 6.9 x 39 x 29 /1000 = 7.6V volts lost at full load (remembering that it will be more than this in practice as the current is greater than the max tabulated value for the cable anyway).

(if someone wants to do the FULL calculation then feel free, but I'm still only on my first coffee....)

So, irrespective of what's going on at the supply end, it's not a great install to the garage.

Now, if the property is at the end of the chain, is old-ish and relatively small then it could easily be the case that when the DNO put the supply in they reckoned on a max demand of only 50-60A and cabled accordingly. Therefore, ANY sudden large current draw has the same implication to the supply cable - we know already that we get at least a 7v drop just on that one circuit alone. The reason it appears to the eye just on one lighting circuit is simple - your fridge/freezer/TV/hairdryer are capable of still working without telling you, whereas as others have said, the eye will notice the tiniest flicker of light.

I'm out in the sticks in Somerset and it's very common in a large number of properties to see the lights in a kitchen 'twitch' when you turn on the kettle, and it's nothing whatsoever to do with the internal installation.

 

[spark 68]

Max rating of 6mm/3 SWA buried is 38A. 9.5kW @ 240V = 39A,

That is for 3 Ph, normally the earthing conductor will not be carrying current (third conductor), I make it 46 A from the table on single Phase.

On my second coffee :ack2: