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.

 

[Engineer54]

Not sure where your all getting your information from, as to plug top/FCU fuse protection. Fuses within these accessories are there purely to protect the flexible cable connected to them. Protecting the equipment in any way is purely coincidental!!

If the manufacturer of an appliance requires a form of OL device to protect components within an appliance, then it should be provided by the manufacture. So in essence, the installation circuit wiring is protected by a suitable rated MCB, Fuse etc, located in the CU/DB. Appliances with a manufactures factory fitted lead/flex with a fusible plug top or is connected to the power source via a fusible connection unit, the fuse is protecting the lead flex, ...Not the appliance!!

 

[Guest111]

Not sure where your all getting your information from, as to plug top/FCU fuse protection. Fuses within these accessories are there purely to protect the flexible cable connected to them. Protecting the equipment in any way is purely coincidental!!

If the manufacturer of an appliance requires a form of OL device to protect components within an appliance, then it should be provided by the manufacture. So in essence, the installation circuit wiring is protected by a suitable rated MCB, Fuse etc, located in the CU/DB. Appliances with a manufactures factory fitted lead/flex with a fusible plug top or is connected to the power source via a fusible connection unit, the fuse is protecting the lead flex, ...Not the appliance!!

Normally in the installation manual for the boiler it states that the appliance must be fed via a 3A fuse and a double pole isolator with a minimum contact separation of 3mm.

 

[SirKit Breaker]

You did the right thing. The MCB is there to protect the cable, and the fuse in the spur is there to protect the appliance flex, but by default is will also protect the appliance.

I agree, the 6a MCB would most likely trip before the 13a fuse, but by fitting the correct fuse you have it all covered.

And as an earlier post stated, you will maintain any warranty.

Cheers.......Howard

 

[clematis432]

Agree with Engineer54, in my particular boiler the PCB,relays,etc (24V circuit) are protected with a 100mA fuse and the pump,fan,solenoids,etc (240V circuit) are protected by a 2A fuse. (both boiler manufacturer,s fuses). This surely puts to bed the idea that the 3A/13A fuse, & 6A mcb protective devices forementioned are not intended to protect the appliance, only the circuit cabling. Nicsin02 posted a worthwhile topic, though don,t think anyone would disagree that the 3A fuse is more appropriate.
Congrats to lively and knowledgeable participants in this forum,as an ex leccie, I love to browse it.

 

[D Skelton]

in my particular boiler

This surely puts to bed the idea that the 3A/13A fuse, & 6A mcb protective devices forementioned are not intended to protect the appliance

In your particular boiler no, you're probably right, however some of the cheaper boilers come with only the 3A fuse in the plug to protect them. As do most appliances.If fuses weren't meant to protect appliances as well as the cable, then every appliance utilising flex with a csa of 1mm or over would automatically come with 13A fuses?? But they don't do they? In fact, if I'm not mistaken, most boilers are fed with 1mm flex so surely a 13A fuse would be ok as it protects the cable feeding it??Somehow I think the legal requirement to protect boilers with 3A fuses has got more to do with protecting the boiler than the cable.