Boost function tripping electrics.

[Lmiln25]

Hi all,

 

I have moved into a house that has a MVHR unit. Said MVHR unit doesn’t look like it has ever ran. Forgive me if I’m not using the correct terminology but I believe it is spurred of the bathroom light switch (pictured below). Every time I get the MVHR running (change 3A fuse) then turn the bathroom light on, upstairs electrics trip. I wonder if it’s as there is a boost function in that bathroom? A possible solution I had come to was to remove the boost function. 

 

If anyone has had a similar situation or could advise I would be extremely grateful.

 

Thanks!

[ProDave]

WHAT trips?  MCB?  RCD?  RCBO if you don't know post a picture of what is tripping.

 

Does the 3A fuse also blow each time it "trips"

 

What is the function of that switch show?  Is that the bathroom light switch?  It looks to be switching two live cables.  Try removing one of the right hand  cables (and terminate it in a connector block for now) and see if that changes anything?

[Lmiln25]

1 hour ago, ProDave said:

WHAT trips?  MCB?  RCD?  RCBO if you don't know post a picture of what is tripping.

 

Does the 3A fuse also blow each time it "trips"

 

What is the function of that switch show?  Is that the bathroom light switch?  It looks to be switching two live cables.  Try removing one of the right hand  cables (and terminate it in a connector block for now) and see if that changes anything?

Hi Dave,

 

Thanks for your reply.

 

RCD trips, fuse blows every time it trips.

 

It’s the bathroom light. 
 

Remove and terminate one of the cables circled in below pic?

 

thanks 👍

[Nickfromwales]

Supply N should be a neutral, currently it is shown as a brown cable eg a possible live conductor. This is very odd, but had been seen before e.g. where brown is a neutral but it should have been sleeved with blue sleeving to denote it is a neutral conductor.

 

Conversely, the cable in load N is a blue cable which has been sleeved to suggest it is a live conductor! A proper mess in other words.

 

Can you confirm which cable is the one going to the MVHR unit?

 

The switch you show must be where the 3a fuse resides, which is replacing a generic 1G1W switch (see pic) usually used to turn a bathroom light on/off in a standard lighting circuit.

 

 

 

What may be happening here is a local ring main has been taken to this switched fused spur (which is now your bathroom light switch), with a 3a fuse in it. Is that correct and the switch is not as I suspected eg isn't just a 20a switched isolator? A light circuit cannot be fed from a supply that offers more than 6a from a breaker in the fuse board, hence the need for the bathroom light feed to be capped by the 3a fuse. This should now be replaced with a 5a fuse.

 

You need to identify the L & N coming into that switch first, the cable that goes on/off when you turn the circuit breaker on/off in the fuse board, that's the very first job. Which breaker turns this on/off, and what is it rated and labelled as, please? This cable needs to go directly to SUPPLY L & N of the fused spur. I assume it goes to the connector block instead, currently, but then you should be seeing a jumper cable, in blue, that goes from the connector block to the SUPPLY N terminal. This would be wrong anyways. I suspect the N for your bathroom light is picked up in that connector block, whereas it MUST be picked up from the LOAD N terminal ONLY.

 

The cable from this location to the MVHR unit needs to be terminated into another switched fused spur, with a 3a fuse in it. This cable should go into the following: Brown (L) into the SUPPLY L terminal (so now two cables in that one terminal) and the Blue (N) into the SUPPLY N terminal. The cable which goes to the bathroom light, should be going into LOAD (1 x brown and 1 x blue per terminal), which gives the switched 5a feed to the light only.

 

Basically, you need to disconnect everything here and start again. First question is, obvs, can you do this and do you feel competent to do so? If you're not 100% comfortable with doing this, with guidance, then DO NOT touch it any further, and get an electrician in for an hour to sort it.

 

"Death is permanent", please try to avoid this  

[ProDave]

5 minutes ago, Nickfromwales said:

Supply N should be a neutral, currently it is shown as a brown cable eg a possible live conductor. This is very odd, but had been seen before e.g. where brown is a neutral but it should have been sleeved with blue sleeving to denote it is a neutral conductor.

I suspect this is switching 2 different live supplies?  as aluded to perhaps trying to boost the mvhr when the bathroom light is on

 

1 hour ago, Lmiln25 said:

Remove and terminate one of the cables circled in below pic?

Yes try that.  If my suspicion is right the bathroom light will work but not the boost function.

 

Let us know the results.

 

If you get nowhere with these simple tests I suggest you get a competent electrician as it sounds like it has been wired in an unconventional way by someone who did not understand what he was doing and failed to find what was wrong and left it with a blown fuse and hope nobody noticed it was not working.

[JohnMo]

Maybe it's a volt free contacts that does boost? And a numpty got his hands on it.

 

What is your MVHR unit - make and model?

[Nickfromwales]

31 minutes ago, JohnMo said:

Maybe it's a volt free contacts that does boost? And a numpty got his hands on it.

 

What is your MVHR unit - make and model?

Crikey. 
 

Is that switch a boost for the MVHR?! 
 

Expect the unit electronics to be nicely toasted if so…..unless it’s mains to a zero volt relay at the unit.

 

The plot thickens.

 

You can add a boost with a quinetic ‘momentrary’ wireless (RF) switch, with ease. 

[Nickfromwales]

Am I wrong in thinking that this switch is where the 230v supply for the MVHR is gleaned from?
 

Or does the MVHR unit get its mains supply from elsewhere? 

[ProDave]

5 minutes ago, Nickfromwales said:

Am I wrong in thinking that this switch is where the 230v supply for the MVHR is gleaned from?
 

Or does the MVHR unit get its mains supply from elsewhere? 

That bit we are all in the dark about.  

 

Trying to follow the wiring, there appear to be 4 cables in total.  3 of them appear quite normal all with neutrals linked in a connector block. Two of them are feed looping in and out in the L Supply terminal.  The third being the L out to the bathroom light in the L Load terminal.

 

The 4th cable is the mystery one.  I think it is intended to be a volt free boost switch connected to the N supply and N load terminals.

 

Without doing some testing or finding out where all the other ends go it won't be easy to solve.  As I already said I suspect it has never worked properly. 

[Nickfromwales]

1 minute ago, ProDave said:

That bit we are all in the dark about.  

 

Trying to follow the wiring, there appear to be 4 cables in total.  3 of them appear quite normal all with neutrals linked in a connector block. Two of them are feed looping in and out in the L Supply terminal.  The third being the L out to the bathroom light in the L Load terminal.

 

The 4th cable is the mystery one.  I think it is intended to be a volt free boost switch connected to the N supply and N load terminals.

 

Without doing some testing or finding out where all the other ends go it won't be easy to solve.  As I already said I suspect it has never worked properly. 

Yup. 
 

Some Agatha Christie shit going on there methinks.

[Lmiln25]

Hi thanks all for the answers, I really appreciate it even thought it’s out my depths I think I have got my head slightly round it. 
 

I opened up the junction box next to the unit and found this (pic below). This should’ve been the first thing I done which I now realise… sorry.

 

The unit is getting its power elsewhere. The brown cable at the bottom is coming from the light switch in for the bathroom. I’m assuming this is the boost function and is what is tripping the electrics and blowing the fuse (fuse box above this junction as shown on picture). 
 

I feel confident that this is the case and am hoping you far more informed people are going to tell me otherwise?

 

If this is the case, could I terminate the brown cable coming up through the bottom of the junction next to the unit? (Boost cable). 

 

Hope I make some sense, thanks.

[Nickfromwales]

1 hour ago, Lmiln25 said:

The brown cable at the bottom is coming from the light switch in for the bathroom.

OK. Unless I'm going colour blind, the cable coming up into the bottom of that junction box is grey?

 

Next, it is defo not coming from the bathroom switch, as the cores in that cable are: Black, Brown and Grey + bare copper earth (green /yellow sleeve over it).

 

So, the image you show isn't anything to do with the switch in the bathroom as the cable types are completely different? This would more likely go to the extractor fan with a timer in it?

 

Keep looking  

[ProDave]

Yes keep looking that is not it.

 

Have you tried that test, disconnecting one of the right hand switch wires?

 

Any pictures where cables connect into the actual MVHR inside views of their junction boxes?

[Lmiln25]

Thanks again guys, I expect to be beaten on the technical side but not on colour! Not sure how I got that wrong, but I did.

 

I think I’m just going to do the sensible thing here let my pride take a hit and get a professional in. As you say, death is permanent.
 

 

[ProDave]

6 minutes ago, Lmiln25 said:

Thanks again guys, I expect to be beaten on the technical side but not on colour! Not sure how I got that wrong, but I did.

 

I think I’m just going to do the sensible thing here let my pride take a hit and get a professional in. As you say, death is permanent.
 

 

If you do that, please come back to tell us what he found.

[Nickfromwales]

1 hour ago, Lmiln25 said:

I think I’m just going to do the sensible thing here let my pride take a hit and get a professional in. As you say, death is permanent.

Indeed. My words are blunt, but honest.

 

Good man.