Fuse size?

[nicsin02]

Today my job was to change fuses in switched fused spurs from 13a to 3a. It was for boilers and the electrician beforehand didn't change them. The circuit is protected by a 6a mcb and fed using 1mm twin and earth cable. Surely if the cable is protected by a 6a mcb then there is now risk of the cable burning out because it would obviously trip rendering the 13a fuse pointless? So my question is would it have been safe if it was left as a 13a in the spur? Thanks

 

[tomo8163]

hi there . my answer would be yes , it is safe as the 6a fuse protects all of the circuit , but bad practise to do so. At the end of the day , if you test a ring circuit do you check every plug unplugged to see if the fuse rating is correct . probably not. the heating engineer should ideally be the one to check the fuse rating at the spur.

 

[Octopus]

Not fitting the appropriate value of fuse in the fuse connection is dam lazy and a sign of poor workmanship.

You did the right thing changing them as its good practice.

 

[clematis432]

Speed of operation between a 6A mcb and a 3A fuse would be (without looking it up) negligible. So in my opinion,theoretically, the fuse did not need to be changed. Regulations probably say it should be changed. Good practice ? that,s only a viewpoint.

 

[amo]

The cable is protected upstream, so it still protects the cable even with the 13 amp fuse, bit pointless really :/

 

[rich.250]

The fuse is the FCU would be to protect the equipment. And the cb to protect the cable.
Also it should provide discrimination so The local device operates before the entire circuit.
You definitely done the right thing.

 

[dagrat]

Totally agree with rich, the local fcu fuse is purely to protect the boiler and is a requirement of the manufacturer and the warranty.

 

[HandySparks]

It's quite likely that the boiler instructions or those of the heating controls say the supply should be fused at 3A.

Edit: As they said above ^^^^

 

[clematis432]

The fuse is the FCU would be to protect the equipment. And the cb to protect the cable.
Also it should provide discrimination so The local device operates before the entire circuit.
You definitely done the right thing.

Would the local device operate before the entire circuit protection ? (3A fuse V 6A mcb) Don,t think anybody could say or prove that. I know you are quoting regs, and rightly so, but the originator was posting a theoretical question and is theoretically right.

 

[rich.250]

In the event of a fault I tend to see the local fuse to bs 1362 operate first in TN installations.
If TT, then it will not protect against an earth fault. Like any device. Apart from a Rcd/RCBO.

In theory the max zs Is less than half for a 3 amp 1362 so it should operate first.

 

[D Skelton]

I've seen PCBs burnt out because they were protected by 13A fuses instead of 3A, even on circuits protected by 5A 3036s.

In the event of a short circuit within an appliance it's anyones guess which of either a 3A 1362 or a 6A B type would trip/blow first. However, in the case of an overload which can be caused by something as simple as a loose connection within the FCU (as experienced first hand), if the FCU is fitted with a 13A 1362 the 13A won't blow, and by the time the 6A B type trips (if it ever does), the PCB is already fried. Also, as most boilers are fed via FCUs on 32A/20A circuits, if protected by a 13A 1362, there ain't no chance on this earth that it's gonna blow in time to save anything should a fault occur within the boiler. The only way to protect a boiler in the event of an overload is to fit a 3A 1362.

I believe it's also a requirement in the Gas Safe regs to protect a boiler with a 3A overcurrent protective device. As I was told once before, it ain't just our regs we have to work to!

 

[Fin170]

If the circuit was already protected by a 6amp MCB.....why did they use a fuse spur instead of a double pole switch?

 

[D Skelton]

If the circuit was already protected by a 6amp MCB.....why did they use a fuse spur instead of a double pole switch?

Read my above post

 

[Fin170]

Read my above post

Fair enough, my faith in fuses has never been too strong as everything seems to melt burn and/or explode before they think amount melting :lol:

 

[Guest111]

it,s usually the manufacturer that specifies a 3A fuse in the spur and if not fitted they can invalidate a warranty claim.When I was wiring heatings for B G a few years ago I used to pick up a box of fused spurs and change the fuses in them all to 3A it only took five minutes and was one less thing to do when you got to site.