RCBO tripping with SWA extension to shed

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rangorgubbins

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Hi all,

Hoping you can offer a novice some advice.
We've had the house re-wired with a new fuse box with RCBOs. Our electrician left the build a while ago and has sadly passed away since. Before he left he didn't finish connecting up the SWA cable running to the shed, so I've done it using some very helpful guides online about how to connect SWA properly with earthing the armour sheath.
I've connected the SWA to a new RCBO in the house and to the old fuse box in the shed which has MCBs. It is working ok except that if I turn on some appliances the RCBO in the house trips. Appliances up to 710 W are ok, but i tried a vacuum (not sure on watts but presume high) and my table saw (1500 W) and it trips the RCBO. The MCB in the shed doesn't trip though.

How do i go about trying to problem solve this? Would by-passing the MCB in the shed make any difference (is that even a good idea?)?
If i connect those high wattage appliances to other sockets in the house they do not trip their RCBO.

Grateful for any suggestions.
Thanks!
 
The RCBO the shed is connected to is rated at 30mA B6.
I've just realised I'm an idiot. The 'B6' bit presumably means the amps, so I've wired the shed to a 6 amp fuse. No wonder it trips!
So i guess i need a B16 or something?

What does the 30 mA refer to? Is that something to do with power spike that triggers it to trip?
 
rangorgubbins said:
I've just realised I'm an idiot. The 'B6' bit presumably means the amps, so I've wired the shed to a 6 amp fuse. No wonder it trips!
So i guess i need a B16 or something?
Click to expand...
That's correct.
As you have fuses in the shed, you could have a higher rating but it depends on the cable, although it still might trip with or before the fuse.
What size is the cable?

What does the 30 mA refer to? Is that something to do with power spike that triggers it to trip?
Click to expand...
That is the minimum current leakage that will make it trip; it can be anything between >15 and 30. That is the current that might be flowing through you instead of the wiring.
 
Thanks for the info.
Its a 3 core SWA with total outer diameter around 15 mm, not sure what the inner cable widths are but can check it tomorrow.
 
Thanks Chivers67, you are spot on. I checked the rest of the roll this morning and its 6mm cables.

The run is less than 25 m, probably somewhere between 15 and 20 m. It's buried in the house (under the screed), then in a conduit when it goes outside until it goes inside the shed.

One more thing I'd liek to check with you guys. I didn't have the rubbery sleeve that is supposed to go over the SWA cable gland connecting to the fuse box in the house. I just wrapped it in electrical tape as I figure its only an earth connection so should be perfectly safe. May not look the neatest but is that OK?
 
What RCBOs Ratings are spare in your Consumer Unit? Well if you haven't got the Gland shroud it's not disastrous as it's earthed and insulated but the tape might go gooey eventually and start to fall off you could buy one for 50p but it would mean taking it out and Terminating it again.
 
There's a 32 and a 16 spare, but they are halfway along the consumer unit, and i found it really difficult to get the RCBOs off the copper live bar thing (looks like a big comb) to wire them up. I can't find an easy way to get at the clasp at the back of the RCBO to release it from the copper live bar. I may just buy a new 16 amp RCBO and swap it for the 6 amp.
 
You should be ok with a 32amp feed to the SWA with that length of 6mm. You need to remove the "comb" to get the rcbo's etc out. Not much risk providing the main isolator is off. There will be something sticking out at the bottom at the back of rcbo's etc with a screwdriver slot - used to lever down the catch away from the din rail it's clipped to. They can just be pushed back on - engage the top first.

Actually I am sure more than 32amps could be used but best to be safe rather than sorry and it needs checking with sums. 4mm would also be likely to be ok. :( I've been doing sums on 40m for when ours is done.
 
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