Type B / C

[AndyPRK]

Can someone give me a domestic example of where they would use a Type C RCBO rather than a Type B ?

For example a wylex 6A or 32A

Thanks.

Would a type C not trip when a bulb blows ?
Would you use these for a washing machine / fridge ?

 

[sparkyspike]

I fitted a 6A C type RCBO only yesterday, for a hiwire lighting circuit with a 300VA toroidal transformer on it. A B type would trip every time you turned the light on.

 

[Albert]

you would use type C or even D when you have a current rush when starting a process. I used D in commercial and only ones in domestic, because it was required buy the manufacturer.

As a principle the only thing that might prevent you from using type C is the Zs, the allowed Zs for type C is about half of type B, if you measure your Zs in the circuit and it is low enough to allow it to trip in case of a fault you can use it. I always use (for lights) 1.5mm2 cable and never had a problem of Zs above the limit.

The allowed Zs for a circuit with 6A CB is about 3 ohm. You should look for other issues on the circuit if you cannot get 3 ohm for Zs.

 

[ban-all-sheds]

A C32 needs a Zs of 0.75Ω for a 400ms trip...

And I would have thought that the chances of a D ever being feasible were very slim indeed.

 

[Click-Sure]

I would only expect to see a Type D (or Type 4, for that matter) in a commercial or industrial property with a much larger supply (and proximity to the substation) than a domestic property.

Allthough as Albert says, its not impossible.

 

[GaryMo]

A C32 needs a Zs of 0.75Ω for a 400ms trip...

0.72Ω (230/(32x10)) though in reality this should be subject to 80% as per Appendix 14.

 

[Adam_151]

Spot the guy who hasn't got a copy of the red book yet

 

[ericmark]

Not sure why one needs Red book to work out permitted Zs?
Type B = 5 x Thermal value
Type C = 10 x Thermal value
Type D = 20 x Thermal value
So a C32 will trip at 320A and at 230V that will require 0.72 ohms pure maths no need for Table 41.3.
Change volts to 240 then Zs would = 0.75 ohms
But with RCD the time for earth fault trip is catered for by RCD so it is the PFC between Line and Neutral which becomes important not Zs and we need it to trip before the cable over heats which will be covered by the thermal part of the trip. So assuming an RCD is used why should a D32 not be used?

 

[ban-all-sheds]

Spot the guy who hasn't got a copy of the red book yet

No - spot the guy who couldn't be @rsed to look up the 230V figures, and just copied the 240V one from here, knowing even as he did so that someone would inevitably make a smart-arse comment...

 

[ban-all-sheds]

So assuming an RCD is used why should a D32 not be used?

Because with a TN supply an RCD is not a get-out-of-jail-free card designed to extricate you from a too-high EFLI?

 

[plugwash]

But with RCD the time for earth fault trip is catered for by RCD so it is the PFC between Line and Neutral which becomes important not Zs and we need it to trip before the cable over heats which will be covered by the thermal part of the trip.

While it may be just about compliant with the regs (I'm not expert enough to know for sure either way) it is certainly very bad practice IMO for a couple of reasons.

1: Generally it is safert if shorts are dealt with quickly, not left sparking for ages.
2: RCDs are nowhere near as reliable as overcurrent breakers.

 

[ericmark]

I may be wrong and maybe you do need a trip to work within a set time?
Table 41.1 does not say what has to disconnect within times shown and RCD will disconnect in under those times. See Note 2.
On my reading the only place where you can't used a RCD to comply with times is with a TN-C system which we can't use anyway.
The impedance between line and neutral will still be important as one has to comply with volt drop.
I will agree that it is better if the MCB will open on it's magnetic part of the device but it is no longer required by the regulations to open due to the magnetic part operating where RCD protection is required.
If I have made a mistake sorry but please quote where it says the time must be satisfied by a MCB and not a RCD.
As to being reliable a RCD tester ensures the RCD works with correct current and time but we do not test the MCB so where those Chinese fakes have slipped the net where they are nothing more than switches the whole idea of relying on MCB's is flawed. Think I would prefer to rely on something I had tested rather than something we hope was factory tested?
And anything sparking far more likely to trip RCD than MCB!

 

[hairyben]

I've fitted C types to garages with half a dozen fluorescent lights before now. Probably not necessary but elegant in design. Also needed a D32 for a heat pump recently You know how hard it is to get a D? Rexel only wanted to sell me a box of ten

 

[davelx]

So assuming an RCD is used why should a D32 not be used?

Because with a TN supply an RCD is not a get-out-of-jail-free card designed to extricate you from a too-high EFLI?

Although the ECA guide to the 17th edition suggests that it is:

"C 4.6.5 Irrelevant ELI specification

A common source of misunderstanding is that of either specifying or measuring values of ELI where a circuit also has an RCD fitted. ELI measurement under these circumstances is a futile exercise"

 

[Click-Sure]

Rexel only wanted to sell me a box of ten

Did you get a free stapler or hole punch with that hairyben?