RCBO Type

[steviez]

Hi,

I am working on a job where there is a 3 phase board with no RCD / RCBO protection on it. Its an old type Square D Quick line board so the breakers are not ready available at local wholesalers. Would it be OK to put a type C 32A RCBO in place of a type B breaker for a ring main?

Thanks

 

[Spark123]

What is the Zs of the ring final circuit?

 

[riveralt]

Your problem will be the Zs figure. Type B @32Amp is 1.16 while type C is 0.58. But if you are working on this type of board you should know that anyway from BS7671 or the OSG.
Since you appear to be in learn mode what is the difference between type B and C?

 

[sparkticus]

Hi,

I am working on a job where there is a 3 phase board with no RCD / RCBO protection on it. Its an old type Square D Quick line board so the breakers are not ready available at local wholesalers. Would it be OK to put a type C 32A RCBO in place of a type B breaker for a ring main?

As others have said you will need a low Zs to comply with a type C 32amp.
In any case it makes no technical sense to fit a type C on a ring final.

Why do you actually need to fit an RCBO at this point in time?

 

[Spark123]

There is nothing technically wrong with using a type C breaker on a ring final circuit, that is of course as long as it complies with max Zs etc.

A quick google search shows type Bs are available from here: http://uk.rs-online.com/web/p/produ...132b03_Broad&gclid=CMza1IbJlaoCFdkz3woduX810Q
(you may need to re-mortgage your house )

 

[sparkticus]

There is nothing technically wrong with using a type C breaker on a ring final circuit, that is of course as long as it complies with max Zs etc.

OK I admit I overstated a little but have you ever had to fit a type C to a ring final?

PS, I am trying not to give any clues to the op so that he and he alone answers the question that riveralt posed.

 

[Adam_151]

OK I admit I overstated a little but have you ever had to fit a type C to a ring final?

Quite common to need to do so in computer labs, a type B will often trip out upon power restoration after an interuption and will require the SMPU load be removed before reseting, fitting type C breakers to your ring circuits reduces the issue; it can still occur with large numbers of computers but then design comes into it, you also dont want too many pcs per circuit

Also normal to try and avoid RCD protection to such areas and provide separate general purpose and cleaners sockets which are RCD protected

 

[mfarrow]

There is nothing technically wrong with using a type C breaker on a ring final circuit, that is of course as long as it complies with max Zs etc.

Does it really matter? You've got an RCD in there, and their speciality is working with high Zs values.

In this instance the choice of either B, C or D curve is acedemic to ADS, and more to do with the equipment to be connected and risk of inrush. Of course it is always desirable to use the lowest denomination you can get away with, but some manufacturers of RCBOs (e.g. Merlin GeSchneider) don't give you the option.

 

[sparkticus]

Does it really matter? You've got an RCD in there, and their speciality is working with high Zs values.

There is no RCD function at the moment. The op is looking to replace the MCB with an RCBO.

But he is only fitting a type C because a B is not available. I just think this is bad engineering, fitting a type C (when there is no call for a type C) where you may not have a low enough Zs for a type C then compensating for that by fitting an RCD (albeit the RCBO takes care of the whole thing)

Surely the right think to do is get hold of the right type B RCBO. I would still be asking: why has the need for any residual current device suddenly become necessary anyway?

 

[EFLImpudence]

Does it really matter?

Yes - RCDs are additional protection. MCB conditions must still be met (unless not possible).

 

[flameport]

There is also the issue of disconnection times for line-neutral faults, for which an RCD will do absolutely nothing.

 

[steviez]

Problem solved now... I have managed to find some type B ones. Thanks for all your help guys

 

[sparkticus]

Problem solved now... I have managed to find some type B ones. Thanks for all your help guys

Good, that is the right solution. But why do you need to fit an RCBO anyway?

 

[ericmark]

There is also the issue of disconnection times for line-neutral faults, for which an RCD will do absolutely nothing.

This is something I have questioned. Looking at 442.5 it seems we may need to have a 5 seconds disconnection time but that does not seem to be cut and dried.

In real terms a line - neutral fault can't really over stress the cables as the thermal part of the MCB will disconnect before insulation on the cables is overheated. The magnetic part is to ensure disconnection speed where the overload is 5, 10, or 20 times design current depending on if B, C, or D type.

The fact that a RCBO has a magnet trip incorporated would suggest time is still an issue.

It is easy to look at 543.1.3 without realising this whole section is on protective conductors so with an RCD is not really required.

With 411.3.2.2 NOTE 1: Disconnection is not required for protection against electric shock but may be required for other reasons,
such as protection against thermal effects.

NOTE 1: Where an RCD is used for earth fault protection the circuit should also incorporate an overcurrent protective device in
accordance with Chapter 43.

434.5.2 does seem to suggest as K is worked out for 5 seconds that the 5 seconds may apply to line - neutral faults.

The only thing which does seem to present a problem is spur length where the loop impedance is above the limit required to open the magnet part of the trip. 433.2.2 seems to limit spur length to 3 meters but it is by no means clear if this is dependent on the line - neutral impedance or not.

So I question where it states the line - neutral disconnection times need to be within a set time which would only be satisfied if the line - neutral impedance was within the limits needed to trip the magnetic part.

With B type RCBO's this does not really matter as the volt drop limits and the magnetic impedance required are nearly the same so we would of course measure the line - neutral impedance to ensure the volt drop is within limits.

But please do explain where time for line - neutral or line - line with three phase faults need to be within the time only afforded by using the magnetic part of the trip?

 

[steviez]

Problem solved now... I have managed to find some type B ones. Thanks for all your help guys

Good, that is the right solution. But why do you need to fit an RCBO anyway?

The board in question is a 24 way three phase one and it supplies sockets to a kitchen area, coffee shop area and lots more. The socket outlets need to be protected right....