Fused Spur for MVHR with Boost

[TimCx]

I am about to wire my new MVHR, the attached lead is 4 core with L, N and Earth and an additional black cable that if energised (at 240v) will switch on the boast function.

 

The boost wiring is connected to switches outside the wet rooms, these were (in the old system) connected to the live from the unit and if any switch was closed then the black wire with be at 240v and will initiate boost, I wont be using this boost initially want to be able to add it later. 

 

What is the correct way of wiring the unit for isolation, the unit wiring diagram looks as though it should be connected to a 3 pole fuse spur but I have not been able to find one (see attached)

 

All wiring will be checked and signed of by a qualified electrician.

 

 

 

[andyscotland]

You can get fused 3-pole fan isolators e.g. https://www.tlc-direct.co.uk/Products/CLCMA3020.html 

although more often the fan is designed and cabled to cope with a 5/6A circuit protective device and just runs directly off a lighting circuit without needing a separate fuse (and/or is internally fused to protect the appliance's own components and connected to the circuit with cable of sufficient size for the protective device).

 

The manufacturer's instructions mention "other means of connecting the unit...see notes on the next page". Do they specify an un-fused option?

 

If the manufacturer specifies that the 3A fuse is always required then you'd need to achieve that. However there's no reason you can't connect a normal unswitched fused spur in series with a normal 3 pole isolator if you can't source / don't like the look of any of the all-in-one options.

 

Personally, I'd put the isolator first and then the fuse plate second so you can easily de-energise the supply (without putting the lights out) for added safety when changing the fuse - in theory that's not necessary but in practice sometimes the fuse carriers can snap & expose terminals so I prefer the safety net of having the supply isolated as well.

 

If the mvhr is accessible you could put the spur plate at the appliance and use a flex outlet plate for the final connection, or you can put it by the isolator and loop in/out with a rear outlet spur plate. You just need the cable between isolator & spur plate to be the same as the circuit supplying it.

 

Of course I would also always advise clearing your proposal with the electrician that will be inspecting & testing as ultimately they're the person that needs to be happy with the details & layout of your circuit.

[TimCx]

9 hours ago, andyscotland said:

You can get fused 3-pole fan isolators e.g. https://www.tlc-direct.co.uk/Products/CLCMA3020.html 

although more often the fan is designed and cabled to cope with a 5/6A circuit protective device and just runs directly off a lighting circuit without needing a separate fuse (and/or is internally fused to protect the appliance's own components and connected to the circuit with cable of sufficient size for the protective device).

 

The manufacturer's instructions mention "other means of connecting the unit...see notes on the next page". Do they specify an un-fused option?

Hi Andy Thanks for your reply its very helpful, the other means relates to energising the boost line when not using the MVHS supply to power it. I have added a scan of that page to this comment

 

9 hours ago, andyscotland said:

If the manufacturer specifies that the 3A fuse is always required then you'd need to achieve that. However there's no reason you can't connect a normal unswitched fused spur in series with a normal 3 pole isolator if you can't source / don't like the look of any of the all-in-one options.

Yes they do state 3amp fuse is required and I will ensure thats done.

9 hours ago, andyscotland said:

 

Personally, I'd put the isolator first and then the fuse plate second so you can easily de-energise the supply (without putting the lights out) for added safety when changing the fuse - in theory that's not necessary but in practice sometimes the fuse carriers can snap & expose terminals so I prefer the safety net of having the supply isolated as well.

Thinking about it a bit more as the boost line is energised from the MVHR supply then a 2 pole isolator to isolate the supply from the CU will also isolate the boost line anyway so I should not need a separate isolator for the boost line?  I also found a statement in the 'Warnings and Safety Information section'  of the manual that says 'The unit should be provided  with a local double pole fused spur fitted with a 3A fuse having contact separation of at least 3mm....'

 

[andyscotland]

I don't think the boost line is energised from the MVHR.

 

The first drawing you posted assumes the power supply and boost trigger are on the same circuit. The neutral is shared but the switched live will need to be energised from the switch end e.g. often by looping from a lighting point.

 

The second drawing has the switched live from a separate circuit (e.g. perhaps power supply taken from the socket ring/a plantroom power circuit and the switch trigger run from a bathroom further away). In that case the neutrals need to be split primarily to avoid causing a leakage current that would trip an RCD/RCBO. But the phase conductor is still energised from the switch end.

 

You need to be able to isolate the appliance for mechanical maintenance and the regs for that require isolation in "all live conductors" which includes the neutral.

 

You also ideally need to be able to reliably isolate all supplies simultaneously, so a single isolator covering the power supply and boost line(s). Theoretically you can isolate the power & boost separately so long as you fit a clear & durable warning notice to any enclosure containing live parts from both circuits (e.g. the MVHR and the connection point) explaining how to safely isolate it. But I would always avoid that wherever possible, it's much safer to have a single switch.

 

[andyscotland]

Also worth noting/considering the location of the isolation switch(es). With bathroom fans they're commonly outside the bathroom door, but assuming your MVHR is buried somewhere I would put the isolator by the appliance.

 

If an isolator for mechanical maintenance is not located where the person doing the maintenance has constant sight/control of it, you need a suitable mechanism for them to prevent someone else turning it back on e.g. a switch with a padlock lock-out.

[TimCx]

19 minutes ago, andyscotland said:

I don't think the boost line is energised from the MVHR.

 

The first drawing you posted assumes the power supply and boost trigger are on the same circuit. The neutral is shared but the switched live will need to be energised from the switch end e.g. often by looping from a lighting point.

 

Thanks again Andy, yes I noticed that in the circuit but on the old unit the boost switches were connected to the unit live and the black wire so if any was switched then the black wire was live. There are 7 boost switches and they were wired in parallel to a common point near the unit.

I think I will leave the boost wiring for the electrician as I wont be using it initially.

 

The isolation switch will be next to the unit and visible 

[andyscotland]

Ah ok, that makes sense. Well yes in that case I guess technically you could use a 2-pole isolator and take the supply to the boost switches from the appliance side of that.

 

Equally I doubt a 2-pole will be noticeably cheaper so it might be worth fitting the 3 as that gives you more flexibility for the future.