Shower causes lights to dim

[Chit]

Hi all,

Whenever the shower is switched on, it causes the lights around the house to dim.

I know this is due to the shower using a large amount of electricity and is not really a problem as such, but just wondering is there is anything that can be done to stop this from happening?

Thanks.

 

[RF Lighting]

Not really I'm afraid.

It'll most likely be caused by a small amount of voltage drop on the supply cable to your house as you drop a heavy load onto it.

It's not dangerous or anything to worry about. You may find that it is not as noticeable with a compact fluorescent lamp.

 

[securespark]

is there is anything that can be done to stop this from happening?

Yeah. Don't switch the shower on.

 

[JohnW2]

It'll most likely be caused by a small amount of voltage drop on the supply cable to your house as you drop a heavy load onto it.

This has got me thinking (always dangerous!!).

I have always assumed that the permitted tolerance on supply voltage (230V +10% / -6%) related to the on-load situation. Is that correct? As far as I can see (maybe this is my mistake?), with a TN-C-S supply, the impedance which will result in voltage drop upstream of the cutout will essentially be equal to the Ze . This Ze is, I think, allowed to be as high as 0.35Ω - which would imply a voltage drop of 28A with a 80A load (or 35V with a 100A load). If that's the case, the no-load voltage at the cutout would have to be at least 244.2V for the supply not to fall below 216.2V (230V-10%) with an 80A load. If it were a '100A supply', the situation would be even sillier, because the no-load voltage at cutout would then have to be 251.2V, very close to the permitted maximum of 253V.

Where has my reasoning gone wrong? If it's true that the specified supply tolerances relate to the on-load situation, the only possible weakness I can think of is my assumption that the impedence resulting in a supply-side voltage drop (with a TN-C-S supply) will essentially be equal to Ze.

Kind Regards, John.

 

[ericmark]

That was what I had assumed. No load voltage has to be max to keep to within lower limits with full load.

As to how to stop it there is a firm who make auto-transformers with the claim their device will save you money by correcting the voltage.
However although before the days of the switch mode power supply this may have been true, today most equipment does not use large volt dropping resistors as found in old TV's and even the florescent at least the compact type using a switched mode supply so today it really would not give the savings claimed. All it would do is cause any old audio equipment to hum.

When living in a caravan site with a huge volt drop I tried to make a device to switch the tapping on a transformer to stop the mains hum. But it was not a success. So I started using a battery to power my radios.
The switch mode power supply could not cope with the huge power surges when using a side band radio transmitter. But in most cases the switch mode power supply has removed the problems associated with volt drop.

 

[ban-all-sheds]

It's not dangerous or anything to worry about.

I disagree - it might be, and should be checked out for peace of mind - if there's a poor connection with a high resistance in the cutout/meter/CU then bad things can happen.

 

[bernardgreen]

It's not dangerous or anything to worry about.

I disagree - it might be, and should be checked out for peace of mind - if there's a poor connection with a high resistance in the cutout/meter/CU then bad things can happen.

I have to agree with BAN on this. The voltage drop is caused by the current for the shower passing through an impedance. That voltage drop and the current produce heat in the impedance. Not a ( serious ) problem if the impedance is spread over a long length of cable as the cable will only get warm.

But a very serious problem if the impedance is not spread over a length of cable but is concentrated in a short length of in-adequate cable or defective joint. If that is the case then that cable or joint will get hot, possibly hot enough to be a fire risk.

 

[flameport]

Where has my reasoning gone wrong? If it's true that the specified supply tolerances relate to the on-load situation, the only possible weakness I can think of is my assumption that the impedence resulting in a supply-side voltage drop (with a TN-C-S supply) will essentially be equal to Ze

Nothing wrong with it, however the other items to consider are:

a. When installing large additional loads such as electric showers and storage heaters, the electricity supplier / DNO is supposed to be notified in advance so that they can confirm additional capacity is available.
If people don't, and the voltage sags all over the place after installing that 20kW pottery kiln in the garden shed, the blame is with those who installed it.

b. Where a single transformer supplies a number of properties, such as in most urban areas, the output voltage will continually be adjusted to accommodate the varying loads, so the volt drop issue is mitigated.

c. In rural areas where the supply is provided from a substantial distance away, the method appears to be to ensure the voltage with no load is as high as possible. (In most of the rural properties I have been to, the voltage is 245-250)

For the original question, if the lights have always dimmed when using the shower, probably nothing wrong.
If this dimming effect has recently appeared or got worse, there is probably a fault somewhere.

 

[westie101]

b. Where a single transformer supplies a number of properties, such as in most urban areas, the output voltage will continually be adjusted to accommodate the varying loads, so the volt drop issue is mitigated.

Er nope sorry, the only automatic voltage control is generally at, for example 33kV to 11kV, at any voltage to 400/230V there is no automatic control.

We generally always did and will commission transformers at around the 250V output for single phase to allow for volt drop on the LV network.

The network is designed to take account of say cookers and immersion heaters but storage heaters, electric showers, motors above a certain size, welders of any size and other large fixed loads the DNO needs to give permission for these loads to be connected.

And yes it is not unusual for a shower to dip the lights, it does in this house! (I seem to recall being told that we can usually perceive a 2 or 3 percent dip)

 

[efiste2]

We generally always did and will commission transformers at around the 250V output for single phase to allow for volt drop on the LV network.

Spot on mate. in SP, Primary 33/11kv transformers have tapchangers that will tap up or down automatically depending on the voltage, secondary 11000/415 transfromers have manual OFF line ones so need to be isolated before adjusting manually.... usually set at tap 3 for around 248V output, expecting the LV network to drag it down....many local secondary substations may not even have a tapchanger....

 

[RF Lighting]

It's not dangerous or anything to worry about.

I disagree - it might be, and should be checked out for peace of mind - if there's a poor connection with a high resistance in the cutout/meter/CU then bad things can happen.

It's very rare to see an electric shower that doesn't casue the lights to dim. I fitted hundreds of showers into disabled bathroom conversions for Leeds city council. Every one of those houses / flats must have had a high resistance joint on the incommer.

It's great knowing the theory, but things are different when you get out there working on installations in the real world.

 

[ban-all-sheds]

1) My lights don't dim, and I have a 10.5kW shower.

2) Please read what I wrote with a little more care.

I disagree - it might be, and should be checked out for peace of mind - if there's a poor connection with a high resistance in the cutout/meter/CU then bad things can happen.

Still - better for you to be wrong and not have a potential source of a fire to be checked than me to be wrong and have a few quid wasted on an electrician, eh.

 

[bhm1712]

same here B-A-S i have a 10.5 kw and the lights dont dim.

 

[RF Lighting]

Must just be the electric up north then.

Honestly I wouldn't worry about it.

 

[JohnW2]

Honestly I wouldn't worry about it.

Whether or not one worries surely depends upon the extent of the dimming and, crucially, whether there has been any change (e.g. dimming recently arisen or got worse)?

Kind Regards, John.