domestic sub main question (fusing)

[offshoreguy]

Hi All,

Been called to a customer property for nuisance tripping which i have resolved but when inspecting installation i have found an issue.

The customer has a cabin at the back of the property with a CU inside, fed by 10mm SWA. it has 32a RM, another 32a (still locating its purpose), 16a radial, 6a lighting and a 6a boiler. This is fed from a 32a MCB in the property CU. this isn't sufficient both due to diversity and it physically keeps tripping.

I want to split off the tails in the house but my question surrounds fusing,

I want to put small RCD unit below the CU fed with separate tails just to the cabin. please see crude diagram below of my intentions.


house CU with RCD
/
Cutoff ---- fused switch ----- service connector
\
RCD ----(SWA)-- Cabin CU


my question surrounds fusing of leg to cabin CU, If I put an MCB diversity is gone for the cabin submain, so i'm pondering on what to do, would the fused switch suffice??

NIC were dog toffee when posing.

 

[davesparks]

First question has to be what do the cable calculations say?
There's no point doing anything if the cable is sized for 32A only, you won't be able to increase the OCPD without replacing the cable.
How long is the cable, what is the volt drop?

 

[Strima]

Why does the SWA need RCD protection?

 

[offshoreguy]

@davesparks - So Voltage drop inst a concern i did it on the basis of the 10mm SWA feeding the CU. 1.1% 10 meters SWA for 60a.

@Strima - the SWA doesnt but the circuits CU hanging off it has no RCD protection if I split it from RCD'd home CU.

BTW the reason it is wired like this is because the builder that fitted the cabin was a have a go harry and basically joined the SWA that used to feed the garage to the cabin SWA. I think he joined in a wiska under the decking. I wanted to pull up the decking and lay new SWA the cabin but the customer is reluctant to have so much disruption. although I its not a show stoper, I did an earth loop and its reading ok so Im not overly concerned.
Im just thankfull the garage was fed by 10mm to start with I imagine he would of joined no matter what the cable was.

 

[pc1966]

One issue you have here is the rating of the sub-main

This is fed from a 32a MCB in the property CU. this isn't sufficient both due to diversity and it physically keeps tripping.

If it is the MCB tripping, and it is not related to downstream circuits tripping (i.e. faults) then it sounds a lot like the MCB is overloaded and you need to fit a larger OCPD, ideally a fuse as you suggest so you get selectivity with MCB trip profiles.

But are all of the feeds off the sub-main rated to be protected by the likes of a 50A fused-switch? The dodgy/unknown joint under the floor would worry me a bit.

 

[davesparks]

I wouldn't entertain doing any work without inspecting, and probably replacing that joint.
[automerge]1597917892[/automerge]

want to put small RCD unit below the CU fed with separate tails just to the cabin. please see crude diagram below of my intentions.

Well that sounds like a terrible plan, and I'd say it doesn't comply personally.

The RCD protection should be at the CU in the outbuilding to minimise inconvenience whne a fault occurs at the very least.

 

[offshoreguy]

@pc1966 - indeed i was thinking 50a fused switch and RCD to sub main.

@davesparks - i am aware its not the best situation but other thank forcing my way to replace the CU in the cabin and jacking up the decking while the customer is in bed Its not my property.

I want to leave this property as safe as possible under the circumstances. I wouldn't say it would not be compliant due to the inconvenience if the CU is protected it doesn't matter where.

I have asked times to lift the decking and refit the CU so I could RCD above the unit but her answer remains the same.

I could hands up walk away but my conscience prevails.


soooo

50 amp fused switch ---to----- RCD-----SWA----submain CU


to what ive read is what I'm going with, unless anyone suggests otherwise ( within my control) - ill try and post a video of the customer installation when i'm on site.

 

[Strima]

the SWA doesnt but the circuits CU hanging off it has no RCD protection if I split it from RCD'd home CU.

Having the RCD at source is poor design, if it trips how easy will it be to access to reset? Having RCD protection in the cabin is the better and safer option.

 

[pc1966]

@pc1966 - indeed i was thinking 50a fused switch and RCD to sub main.

You should check the end(s) of the sub-main Zs to make sure the fuse will achieve an acceptable disconnection time of under 5s.

The upstream RCD should avoid this, but its not really good practice in my view to depend on the RCD for fault clearing, and by designing/verifying for acceptable OCPD fault clearing you are free to move the RCD later to a better position if possible.

 

[davesparks]

@davesparks - i am aware its not the best situation but other thank forcing my way to replace the CU in the cabin and jacking up the decking while the customer is in bed Its not my property.

So make your recommendations of what should be done in compliance with the regulations. If the customer decides they don't want it done then they don't have it done.

What exactly is the job that the customer has asked you to do here?

Are you sure that it isn't something in that joint which is causing the tripping of the 32A MCB?

 

[Baddegg]

What are they using the cabin for that could overload a 32a mcb??.....I’d say the joint box is most likely not correctly ip rated especially given the installation circumstances....ideally rcd at cabin end but obviously you can’t make em...

 

[DPG]

What are they using the cabin for that could overload a 32a mcb??.....I’d say the joint box is most likely not correctly ip rated especially given the installation circumstances....ideally rcd at cabin end but obviously you can’t make em...

Fair point about the load. My summerhouse runs from a 13A fuse with loads to spare.

 

[pc1966]

Is it really a RCBO feeding things? Having a length of SWA and numerous circuits off that would be a good recipe for spurious tripping!

 

[Wilko]

What are they using the cabin for that could overload a 32a mcb?

This was my first thought too .
I have had an MCB that was a bit trigger happy, tripping after a few hours at about 2/3 rated. A like for like replacement resolved the tripping. But I only did that when I established no faults existed and the load was within spec.