Light dims when shower switched on - voltage drop?

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Hi all, I wondered if I could please ask a question re: an electric shower. When turning it on, the bathroom light dims/flickers once. It doesn't stay dimmed while it is on, only as the power button is pressed, so presumably it's not voltage drop but could be wrong. I assume it's safe enough but does anyone know why this would be happening? Thanks
 
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Since the lighting circuit and shower circuit are independent until the consumer unit, unlikely one would see a volt drop with the lights, seems more likely a poor connection some where, I would inspect to be on safe side.
 
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Thank you both. May I ask 2 questions please?
1. Is it safe to wire a bathroom light into a shower circuit, assuming that a poor connection is not the issue? (presumably yes but just checking)
2. Does anything about this issue suggest that the installation was done by someone who isn't an electrician? The reason being that this is a rented property and a few issues have cropped up recently, long story short there was no planning permission and I'm starting to think nothing has been signed off either. Again, probably all good on this one but just trying to get a sense of anything potentially unsafe

Just to clarify it's an electric shower rather than a power shower.

Thanks again.
 
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Since the lighting circuit and shower circuit are independent until the consumer unit, unlikely one would see a volt drop with the lights, seems more likely a poor connection some where, I would inspect to be on safe side.
I'm not so sure about that - what about the voltage at the CU? If one has, say, an external L-N loop impedance of 0.3Ω, then switching on a 40A shower will cause a sudden fall of voltage at the CU of 12V (over 5% of 230V). Whilst that may well not result in any noticeable effect on LEDs, with incandescent bulbs it would represent a sudden reduction in dissipated power of over 10%, which almost certainly would be noticeable, wouldn't it?

Kind Regards, John
 
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Although it's not unknown for a bathroom light to be wired into the shower circuit.
It has become quite common where the bathroom is refitted, including a new electric shower and CU/RCD, but the lighting circuit is not RCD protected, just add an additional 6A MCB for the light. Job'sa good'n
 
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