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Taylortwocities

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I have been to look at a friends flat that he lets out. As result of EICR he had to have the old Wylex fuse board changed for a Fuseboard consumer unit. all RCBO.

This want done by me.
After the electrician left, the tenant complained that the electric underfloor heating was tripping the two 20A RCBOs on the two UFH circuits. He returned and swooped the B20 RCBOs for C20 MCBs. But the problem persists.

as background, each UFH zone is 24v and there are huge toroidal isolating trannys converting 230 to 24v. One is 1.25Kk snd one 2.2KW.

I’ve checked everything, and it all worked fine with the old Wylex, I plan to swop out the C20 MCBs for 20A fuse carriers with 20A BS1361 Fuses as I think the in surge is doing the dirty.

I cannot see a fuse carrier for the Fusebox CU. Does anybody know if they exist and/or if another (I’m thinking Hager) fits the Fusebox Busbar alignment. ???
note: I don’t like mixing makes in a CU, but sometimes.,,,
 
Presumably you can't get a D-curve MCB for this? Or is the Zs too high for disconnection in that case?

Somewhere I found a Hager guide to sizing MCB for transformers and for 2.5kVA they have 63A B, 40A C, or 20A D as suggested options.
 
Another option would be a stupidly high MCB feeding a small metal DIN enclosure where you can put in some fuse carriers. The ones I have used are here and rated to 32A:

And a matching pack of 10 * 20A fuses:

Maybe not as elegant, but saves any bus-bar issues. Or get a fuse carrier first and compare with the Fusebox MCB/RCBO to see if it has the input clamp in the same place & same size, etc, so busbar is going to be fine.
 
Thanks. Just got a photo of the smaller tranny.
Fuse carrier for Fusebox 36E37038-E944-458E-99A5-D464049D670C - EletriciansForums.net
Strange thing is that I can run either circuit and they hold in for a while, then the MCB trips. I can cycle the individual circuits on and off as each has a 13A FCU in front. And MCB doesn’t trip then, so I eliminated inrush as the cause.
It all used to work with the two circuits as spurs - Wylex rewireable from separate 30Aring finals - so I’m expecting 20A fuses to resolve…
I see Fusebox doesnt do D curve, separate box isn’t going to work. so I’ll investigate alignment of Hager v Fusebox busbar.
It’s never easy, is it ?
 
It might still be inrush currents if the primary of the transformer is switched by some thermostat.

I know that toroidal transformers seem to be worse for inrush than some of the traditional EI style laminated things but not sure why (should look it up some time!). What I have done in the past is to fit NTP thermistors as inrush-suppressors but then you need somewhere to put a box and terminals, etc, for that devices. This sort of thing:

However, they are designed to run hot, so they sit at 80-120C when "stable", and any box & terminals has to allow for that. They don't need cooling - kind of defeats their purpose - but we always used ceramic "choc blocks" for fixing them just due to this, and leave a good cm or so of wire between the terminal and device as well. If sleeving use silicone or glass-fibre as PVC won't be happy at those temperatures, but in a sealed box it is not really needed.
 
I see Fusebox doesnt do D curve,
Given their domestic market focus that doesn't surprise...

separate box isn’t going to work.
No room anywhere for a small enclosure?

so I’ll investigate alignment of Hager v Fusebox busbar.
Fusebox MCBs will be badge engineered - have a look at other "makes" with the same appearance.

Never looked at one, but will they accomodate a cable as well as a busbar prong?



It’s never easy, is it ?
If it was it wouldn't be fun.

They say.
 
For some reason I replied without reading this.

Circuit is supply > 20A C curve MCB > 13A FCU > Contactor (operated by Thermostat) > transformer. All wired in 2.5milli T&E.

If there's that FCU, why have high current MCB -> fuse -> FCU? Why not just put in the bigger MCB. It only has to do fault protection, but you could replace the cable with a larger one.
 

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