RCBOs type C ?

[sparkytim]

Hi all

As loop impedance is not an issue (in most TN cases) with RCBOs what are the limitations of type C RCBOs over type B ?

Noticed that pre 17th, Type C RCBOs seemed the norm now becoming not so common, in fact my local Edmundsons was selling off Crabtree type C at

 

[sparkytim]

No one got any ideas then?

 

[Lurch]

Not really sure there's any ideas to be had, type B, type C, check your Zs. Think that about covers it.

 

[Andy™]

less chance of tripping with a high start current load / lamp failure, but thats about it.

since its RCBO, not really any issues with Zs, but in most cases, you could meet Zs of the MCB part

 

[sparkytim]

[quote name='Andy

 

[a1]

then why have they gone over to making Type B when i am pretty sure all the earlier ones were Type C.

They all were, indeed, Type C RCBOs, Mate.

 

[binky]

Yes thats what I mean Zs not an issue as the RCD part looks after shock protection then why have they gone over to making Type B when i am pretty sure all the earlier ones were Type C.

Probably popular demand - everyone is used to type B MCBs so expect type B RCBOs to match. Keeping to type C would make fitting them a bit more attractive if less prone to nuisance tripping by lightbulbs blowing. It's a bit like 25A MCBs - who ever fits them ??? No-one cause it ain't a 20A or 32A.

 

[gselectrical]

Yes thats what I mean Zs not an issue as the RCD part looks after shock protection then why have they gone over to making Type B when i am pretty sure all the earlier ones were Type C.Just wonderd if slower operating threshold could be an issue on over current faults?

Doesn't the RCD have to be double pole for the 1667 max Zs to be used, or am I completely missing something! J

 

[Andy™]

RCD max Zs is to meet disconnection times when other methods dont work - i.e TT systems - nothing to say it must be DP

 

[gselectrical]

Cheers Andy.

 

[kme]

It's a bit like 25A MCBs - who ever fits them ??? No-one

Not quite mate - I fitted 130+ of `em a couple of weeks ago !!!!

And they were a mix of "C" and "B" curve........

 

[Andy™]

what were the circuits?

 

[kme]

Mostly over-rated 32A, picked up on a prior PIAT - commercial Hotel (200+ rooms). The old breakers were type 4, and in order to get Zs within limits, and in some cases current-carrying capacity of cable, the ccts had to be downrated from 30/32 to 25. We ( main contractor & us ) agreed on 25 B/C, dependant on wiring & pre-existing loading, as given by the schedule.

 

[batty]

[quote name='Andy

 

[SPECIAL LOCATION]

Hi allAs loop impedance is not an issue (in most TN cases) with RCBOs what are the limitations of type C RCBOs over type B ?

Noticed that pre 17th, Type C RCBOs seemed the norm now becoming not so common, in fact my local Edmundsons was selling off Crabtree type C at

 

[sparkytim]

On 'TN' installations to comply with BS7671, you must still meet the same Zs values for MCB's OR RCBO's appropriate to the type B,C or D.Read the descriptions at the top of table 41.3 and the (a) (B) and © headings. Page 49, all under 411.4 TN systems.

So for type C you still lose approx half you max cable length.

But (as I think you imply in your post) these lengths are normally too much of an issue with the average Domestic property.

As for why Type B -vs- Type C manufactures...

IMHO it would relate more to the real world applications where they were to be used..

Due to costs and very limited requirement for RCBO's pre 17th in domestic..

Thanks for that but does it not say in table 41.3 Max loop for 0.4 s disconnection for type B, C, and D for 60898 MCBs and THE OVERCURRENT CHARACTERISTICS of RCBOs?

As the RCD part will look after earth leakage type faults

So should we not be checking L-N Loops for RCBOs? ?:|

 

[binky]

Mostly over-rated 32A, picked up on a prior PIAT - commercial Hotel (200+ rooms). The old breakers were type 4, and in order to get Zs within limits, and in some cases current-carrying capacity of cable, the ccts had to be downrated from 30/32 to 25. We ( main contractor & us ) agreed on 25 B/C, dependant on wiring & pre-existing loading, as given by the schedule.

They have their uses, but rarely seen. Could be useful for radial circuits if you are prepared to get close to cable limits on a shortish run