3A mcb's

[stunlawless]

Afternoon everyone,

Usually, manufacturer spec for extractor fans is to be fused down to 3A .

Any body know why I shouldn't use a 3A mcb (plenty of spare ways in this CU !) ?

Fan rating = 20w; inrush current can't be near 3A can it ? I assume a 3A plugtop fuse would be slower than a B type mcb. Do the charts in 7671 cover these dinky little things ?

The CU is a Hager, but they don't do a 3A B type, but in C type they do 0.5, 1, 2, & 3 A, Any thoughts on using a C type mcb with a TT earth (not measured Ze/Zs yet) ?

Alternatively, MK do a 3A B type. Anyone know if a MK Sentry mcb is an easy fit in a Hager CU ?

Thanks for looking, David

 

[ericmark]

Some reasons but not real answers.
1) The popular fuse sizes are 3 and 13 amp and although 1,2,5,7 and 10 amp are also available they are non preferred sizes so likely where a fuse is asked for with any appliance under 650W that it will state 3 amp.
2) Under short circuit conditions fuses will rupture quicker than MCB's will open and some electronic equipment does require semi-conductor fuses to adequately protect the device although this is unlikely to be the case here.
3) In order that the fuse supplying the fan will blow before the main lighting fuse it would be sensible to have some discrimination again because 5A MCB has completely different curve to a fuse this is also unlikely.
4) Something within the fan may burst into flames at any more than 3 amp. Again unlikely and any fault is likely to mean the whole fan is replaced normally there are no user replaceable parts so in real terms the fuse only protects the wiring.

The question has come up many times and one person claims to have phoned the manufacture who had insisted they use a 3 amp fuse but I would think more likely it's all about item 1 and connecting to 5 amp MCB will be no problem. There is only one way to find out. Ask the manufacture. And if you do can you report what he says please.

 

[Spark123]

In anycase, don't fit MK MCBs to Hager boards - only use the correct manufacturers MCBs for the model of consumer unit.
You'll probably find they will spec a 3A plugtop fuse anyway.

 

[kai]

What's wrong with fitting a half amp type C in the CU then?

It is easy enough getting the earth loop resistance down to a low enough value to trip a half amp type C breaker.

 

[securespark]

Eric: I am puzzled.

In my limited experience of appliance faults, a B32 seems to trip before the 1363 fuse blows.

Can you enlighten me?

 

[ericmark]

Eric: I am puzzled.

In my limited experience of appliance faults, a B32 seems to trip before the 1363 fuse blows.

Can you enlighten me?

Yes I would agree but depends why it blows. I direct short circuit and a semi-conductor fuse will blow before the MCB.

I first came upon the problem with a machine which used heaters. The contactor for the heaters was burning out on a regular basis because of the constant on/off cycle. So my boss decided in his wisdom to fit a Semi-conductor contactor instead but this produced a further two problems.

1) If the heaters short circuited it would blow the semi-conductor contactor every time rather than trip MCB and this was cured by using fuses.
2) The emergency stop original acted on the control circuit which now had semi-conductors so not longer allowed and a main emergency stop contactor had to be added.

This has stuck in my mind

 

[stunlawless]

Morning everyone,

Just spoke the fan manufacturer (Airflow).

Asked if I could use a 3A C type MCB in place of a 3A fuse, and they said "yes" without a moments hesitation.

So that's the way forward then !

 

[JonoPashley]

a 3A type C mcb could trip at up to 30A assuming that you can meet the Zs figures. a 3A plugtop fuse will trip at upto 5Aish which do you think would be safer?

 

[stunlawless]

Jono, surely it depends on what kind of fault condition is present.

With a short line-neutral (line-earth taken care of by RCD), then the delay of the C type would mean that the actual current is considerably higher than 3A when it breaks (haven't got the regs to hand, but 30A sounds reasonable).

Not sure what that means for conductor temperature (1.5 mm T&E).

If, on the other hand, it is a gradual overload, then the mcb will trip at 3A, after a small delay, but the fuse will run for a lot longer (do the regs cover BS1362 fuses ?) with a small overload (+50%). What would that do to the fan (cables will be fine) ?

Shan't be back here till Monday - don't think I'm ignoring you !

 

[Spark123]

Jono, surely it depends on what kind of fault condition is present.

With a short line-neutral (line-earth taken care of by RCD), then the delay of the C type would mean that the actual current is considerably higher than 3A when it breaks (haven't got the regs to hand, but 30A sounds reasonable).

A type C MCB will clear within 0.1s at 10x its rated current.
Do Hager do a 3A MCB? If not then there is little point in continuing this approach.

Not sure what that means for conductor temperature (1.5 mm T&E).

If the fault level is up at 30A then the MCB will trip quickly (0.1s) and before the cable overheats.

If, on the other hand, it is a gradual overload, then the mcb will trip at 3A, after a small delay,

No it won't, it will never trip at 3A. IIRC the ruling it it must trip within 1hour at 1.45x its rated current. If the current gets higher, the device will heat up and operate more quickly.

but the fuse will run for a lot longer (do the regs cover BS1362 fuses ?) with a small overload (+50%). What would that do to the fan (cables will be fine) ?

I don't think there will be that much in it tbh.

 

[ericmark]

These fans if the go faulty are a throw away unit. The idea of fuse or MCB is to protect the cable and prevent fire. B, C, or D MCB will all likely do this although the magnetic part may need 60A to trip the thermal bit will still work on 3A which will protect against fire.

 

[stunlawless]

Morning everybody,

Interesting debate, but it's all become a bit academic for me.

Hager do a C type 3A mcb, but I looked closer and it's actually a Wylex CU, and they don't do a 3A mcb.

So, I've put it on a FCU in with a 3A fuse.

More fun trying to put the hole (witha 110mm core drill) through the wall - half way through, the drill stopped and started smoking....

Cheap screwfix ownbrand job - only lasted 5 years.

Thanks for thinking about it, David

 

[Spark123]

Don't know if el-cheapo screwfix drills are rated to core up to 110mm. I think my Makita is only just and so rated to that size.
Word of caution, when core drilling use a drill which has a clutch and a nice long handle, otherwise if it bites it can do you damage.

 

[stunlawless]

Morning spark123,

I guess it's pretty obvious that el cheapo wasn't up to the job, Shame really, it chewed it's way through anything it was pointed at, inlcluding breaking out concrete. Iut doesn't owe me anything !

Anyway, I'm off to the hire shop now....