Electrical interference/leakage

[James94]

Hi all,

I have had an electrician come and change how a bathroom inline fan is controlled and when they had gone I noticed two of the four bathroom led lights are on constantly, very dim. If I turn them on and off and dimmer them they work fine but when off stay dim.

I'll give you a little background to what he's done. The fan was originally on a switch and he's changed how it works by installing a manrose 1361 remote bathroom fan humidity control with timer. He's also added a fan isolator in loft and he's connected it to the new fan which was in loft  manrose mf100t.

So, I've had him back and he thought he sorted it but same when they'd gone it was back the same two dim lights. He's now saying that it's because of the set up and the type of fan installed he's saying we need to buy a new fan that doesn't have an over run, is he correct?

Regards james

[Nickfromwales]

If they’re cheap leds then you may need a snubber (capacitor) across the feed to the lights. 
 

Do the leds only glow when the fan is running, and then go out when the overrun time has elapsed? 

 

Manrose is industry standard stuff btw, so it’s not like it’s made from Ali Express chinesium or worse. 

[James94]

Hi Nick,

there gu10 led bulbs from Screwfix, so not sure about cheapness.

he has mentioned about a capacitor but i don’t really understand it to much.

It’s only started since he’s fitted the humidity sensor, is this what’s causing it as fan is original?

 Im not sure what leds do when running as usually in shower when it comes on 😛 and light usually switched on. But when lights switched off the stay dim.

Is there any tests I can carry out to see what’s going on?

do I need to buy a new fan like electricians said as the one in is new, he says I need fan without overrun built in? Would this cure it?

Regards. James

 

[Nickfromwales]

I think we have found the problem! 
 

Just re-read your first post….

 

21 hours ago, James94 said:

If I turn them on and off and dimmer them they work fine but when off stay dim

Are you saying you have a dimmer switch controlling these lights? If so that needs to go, and the sparky should have told you this. LED dimming causes all sorts of grief, plus you can’t control fans or timers with a variable voltage input such as a dimmer. 
 

Afaic, this needs to be a regular switch, so if you can, change that over and test it again. 

 

 

@James94 also, you need the overrun for when someone’s used the loo, so there’s no reason why these two functions should be working without issue. 

[James94]

Hi Nick,

can’t seem to get my head around what your saying, I’ve always had them on a dimmer switch and had no issues. The humidity sensor and fan are totally separate to the lights, this is why I can’t get my head round why it’s effects the lights. 
I will put a normal switch on and see if it makes any difference but electrician didn’t say owt about that.just said I need new fan.

I’m not bothered about when someone has been to loo, it’s just through the humidity sensor when it comes on. I think this is why  the electrician has said I don’t need a fan with an over run as the fan is powered up until no moisture is sensed,am I wrong in thinking this?

Regards. James

[Nickfromwales]

1 minute ago, James94 said:

Hi Nick,

can’t seem to get my head around what your saying, I’ve always had them on a dimmer switch and had no issues. The humidity sensor and fan are totally separate to the lights, this is why I can’t get my head round why it’s effects the lights. 
I will put a normal switch on and see if it makes any difference but electrician didn’t say owt about that.just said I need new fan.

I’m not bothered about when someone has been to loo, it’s just through the humidity sensor when it comes on. I think this is why  the electrician has said I don’t need a fan with an over run as the fan is powered up until no moisture is sensed,am I wrong in thinking this?

Regards. James

So you have a separate switch for lights, that doesn’t provoke the fan to come on at all? 
 

And a second switch to trigger the fan to start and overrun. 
 

And the humidity stat is permanently powered and switches on / off of its own free will?

[James94]

Hi Nick
 

So you have a separate switch for lights, that doesn’t provoke the fan to come on at all? 
yes

[James94]

Hi Nick

And a second switch to trigger the fan to start and overrun. 

no the humidity sensor powers the fan until no humidity sensed then turns off, I think that’s how sensor works.

[Nickfromwales]

FYI, if you click on text in a post and highlight it, you’ll see a pop up saying “quote selection”. 
 

To ‘mention’ a member, type @ and the first letter of their username and select from the drop down list   

[James94]

Hi Nick

 

And the humidity stat is permanently powered and switches on / off of its own free will?

powered by humidity sensor and goes off when no humidity, I think that’s how it works.

Regards. James

 

[James94]

Just now, Nickfromwales said:

FYI, if you click on text in a post and highlight it, you’ll see a pop up saying “quote selection”. 
 

To ‘mention’ a member, type @ and the first letter of their username and select from the drop down list   

😛 

[Nickfromwales]

1 minute ago, James94 said:

Hi Nick

And a second switch to trigger the fan to start and overrun. 

no the humidity sensor powers the fan until no humidity sensed then turns off, I think that’s how sensor works.

Yes, got that, thanks.

 

I meant that the spark wired to humidistat to be on 24/7, so is likely to just be on the loop in and not connected to the lights. 
 

This means the culprit seems to be the fan.

 

I'd contact the manufacturer and ask if they can provide a snubber and give details of where it should be connected to rid you of the issue.

 

At least this should discount the dimmer / light switching. 

[James94]

Thanks for all your help Nick,

I will try and get in touch manrose tomorrow and try and explain what’s going on and see if they can recommend a solution.

Regards James

[Nickfromwales]

1 hour ago, James94 said:

Thanks for all your help Nick,

I will try and get in touch manrose tomorrow and try and explain what’s going on and see if they can recommend a solution.

Regards James

Cool. Just better to not void the warranty of your new equipment, so see if they have a product to give you. 

[BotusBuild]

IF the humidity sensor gets its power from the lighting circuit, the sensor, the fan and the dimmer switch are now all electrically connected.

The combination of the electronics in the sensor (the NEW item in the circuit), the fan and the dimmer switch may be causing interference along those electrical connections causing a trace current to exist causing the LEDs to glow dimly. Seen this before and it was an elimination exercise to get rid of it. As @Nickfromwales has said try the switch change (the simple cheap first step). After that it's back to the electrician and check what circuit that sensor is connected to.

 

It may have been a better idea to change the fan to one with an integrated humidity sensor e.g.

https://www.powerdiscount.co.uk/greenwood-unity-cv2gip-smart-dmev-extractor-fan-with-timerhumidistat-764-p.asp?msclkid=fcf8825d4a5e11a8765eeb2b0f8bd0b6

[James94]

@BotusBuild

thanks for your advice, I see what you mean by electrically connected, I only went this route as I already had a new inline fan in the loft wasn’t aware that because it has a timer on it it’s not compatible with the humidity sensor(spoke to manrose today).so I have to buy a new fan without timer and see how that goes, I’ll post update when I have it fitted.

Regards. James

[ProDave]

38 minutes ago, James94 said:

so I have to buy a new fan without timer and see how that goes, I’ll post update when I have it fitted.

Don't do that.  Post a picture of the insides of your fan and I am 99.9% certain you can just remove the timer module and your fan will work as a normal fan.

[James94]

@ProDave

thanks Dave,I’ll take a picture tomorrow when I get home from work.

Regards  James

[Andehh]

We have an illuminated mirror with one of those swipe to turn on. 

 

During the night there is the slightest of glows that comes from it, only slightly noticable in the dark. 

 

I'm sure I read somewhere that power cables running long distance in parallel can allow a tiny "cross contamination" which generates just enough to trigger the LEDs. 

 

Sticking it into Copilot AI give other reasons including sharing a neutral etc, might be worth looking through it..... 

 

-----—---------------------------

 

A faint glow from LED lights even when they’re switched off is surprisingly common, and it usually points to one of a few electrical behaviours rather than a fault with the bulb itself. You can think of it as “ghost current” finding its way through the circuit.

⚡ Common Reasons LEDs Glow When Off

1. Residual Current in the Circuit
LEDs need very little power to produce light. Even a tiny trickle of electricity—far below what an incandescent bulb would notice—can make them glow.
- Happens often in older wiring.
- Can be caused by long cable runs acting like an antenna.

2. Backfeed From Other Circuits
If your lighting circuit shares a neutral with another circuit, a small amount of current can leak across.
- Very common in UK homes with multi‑gang switches.
- LEDs are sensitive enough to pick it up.

3. Illuminated or Smart Switches
Switches with:
- Neon indicators  
- LED indicators  
- Smart switch electronics  
…all allow a tiny current to pass through even when “off”.

That tiny current is enough to make LEDs glow faintly.

4. Capacitive Coupling
If your switch is on the neutral side instead of the live side (common in older UK wiring), the lamp still has a live feed present.  
The cable itself can induce a tiny voltage into the LED driver.

5. Poor Quality LED Drivers
Some cheaper LED bulbs don’t fully discharge stored energy when switched off, so they “sip” power and glow for a while.

---

🔧 Practical Fixes (Safe, Non‑DIY)
I won’t tell you to do electrical work yourself, but these are the usual remedies an electrician would consider:
- Ensuring the switch breaks the live, not the neutral.
- Adding a load resistor or anti‑flicker capacitor to absorb ghost current.
- Replacing the switch if it has an indicator light.
- Checking for shared neutrals or wiring faults.
- Upgrading to higher‑quality LED bulbs with better drivers.

---

If you want, you can tell me:
- Whether your switch has a little indicator light  
- Whether the LEDs are on a dimmer  
- Whether they glow constantly or only briefly after switching off  

…and I can narrow it down to the most likely cause for your setup.

 

 

[LnP]

I have some outside LED lights which are switched by an outside switch. When the light switch is in the off position, there is a small red neon light in it which comes on so you can find the switch in the dark. When the switch is in the off position, the outside lights do go off but continue to glow dimly.

 

I'm not an electrician, but as far as I can work out, the reason is as follows. The circuit is wired with no neutral to the switch. The neutral is wired directly from the consumer unit to the lights. The live goes from the consumer unit to the switch and the neon light is wired  in parallel with the switch. When the switch is off (open), the LED lights allow enough current through so that the neon illuminates, but this also causes the LEDs to glow. Obviously when the switch is closed, the current takes the easier route through that and the neon goes off.

 

I suppose that it would have been possible to wire live and neutral to the switch box so that the neon could have its own neutral. That would stop the LEDs glowing but the neon would be on all the time, which I wouldn't mind. I wish the electrician had discussed this with me before he wired up the circuit.

 

A general question to those who know ... I suppose wiring up lighting circuits with no neutral to the box is cheaper and that's why an electrician might do it. I'm sure it complies with the regs doing it that way, but are there other disadvantages apart from my glowing LEDs? Is it better to have a neutral in the switch box as well as a live? What about if e.g. you want smart lighting?

[James94]

@Andehh Thanks for your advice, there's certainly a lot of variables there.

 It's a modular switch it has a dimmer which controls the led lights and a spare switch which used to control the fan. There is no light indicator and only 2 of them glow constantly, the first brighter than the second. I have been told by manrose that the fan isn't compatible with the humidity stat, so going to take picture later and see how simple it will be to remove the timer on it. I'm not sure yet if this will sort out the glowing leds yet.

Regards  James

1 hour ago, Andehh said:

Whether your switch has a little indicator light  
- Whether the LEDs are on a dimmer  
- Whether they glow constantly or only briefly after switching off