RCBO Query

[wreck-it-steve]

Hi,

I need to replace a faulty RCBO in my consumer unit and can only seem to get hold of type C. Is this OK to use in a domestic property? Maybe it will stop the RCBO tripping when a bulb blows?

I have a Merlin Gerin Isobar 4 board and my local wholesaler says they dont do type B for them.

 

[PrenticeBoyofDerry]

I think these are the ones
http://www.expert-electrical.co.uk/...n-Single-Pole-6-Amp-30ma-Type-B/prod_388.html

 

[wreck-it-steve]

I think these are the ones
http://www.expert-electrical.co.uk/...-Pole-6-Amp-30ma-Type-B/prod_388.html[/QUOTE]

They are not Merlin and the line feeds in to the bottom instead of the top like merlin rcbo's

 

[EFLImpudence]

If you are only able to get type C for your board, does that not mean you would already have type C?

 

[wreck-it-steve]

If you are only able to get type C for your board, does that not mean you would already have type C?

No, I have all Square D SQOR's (type B's) installed but I cant find any type B.

 

[echoes]

For a 6A circuit using 1mm² cable and TN- supply, the voltage drop is the limiting factor on cable length, so a C-type RCBO should be fine, (subject of course to a Zs test at the furthest point), but assuming an already compliant circuit, no problems.

 

[PrenticeBoyofDerry]

They are not Merlin and the line feeds in to the bottom instead of the top like merlin rcbo's

your right sorry about that.

you can get them though but cost an arm/leg
if you google
C60HC6R30
you will see!

 

[EFLImpudence]

a C-type RCBO should be fine, (subject of course to a Zs test at the furthest point),

Correct.

but assuming an already compliant circuit, no problems.

Not exactly correct.

The Zs maximum for type C is half that of type B but on a lighting circuit is unlikely to be a problem but should, nevertheless, be confirmed.

 

[echoes]

a C-type RCBO should be fine, (subject of course to a Zs test at the furthest point),

Correct.

but assuming an already compliant circuit, no problems.

Not exactly correct.

The Zs maximum for type C is half that of type B but on a lighting circuit is unlikely to be a problem but should, nevertheless, be confirmed.

Yes - agreed. I was meaning existing compliance in terms of voltage drop would imply compliance in terms of Zs for a 6A lighting circuit. Not to be taken as blanket statement for all current-ratings and cable sizes.

 

[JohnW2]

Not exactly correct. The Zs maximum for type C is half that of type B but on a lighting circuit is unlikely to be a problem but should, nevertheless, be confirmed.

Agreed, what what echoes wrote was:

For a 6A circuit using 1mm² cable and TN- supply, the voltage drop is the limiting factor on cable length, so a C-type RCBO should be fine, (subject of course to a Zs test at the furthest point)

The maximum permitted VD of 6.9V at 6A means a maximum 2*R2 of 1.15Ω - which, for 1mm cable (cpc also 1mm) means an R1+R2 also of 1.15Ω. The maximum permissible Zs for a 6A Type C device is 3.83Ω. That means that if the Zs were too high for a Type C RCBO, Zdb would have to be in excess of 2.69Ω - clearly totally unacceptable for a TN installation.

Kind Regards, John.

 

[wreck-it-steve]

Think I may just stick a 6a MCB in there until an RCBO pops up on ebay.

 

[ericmark]

JohnW2 has given you the reason why at 6A the B type RCBO is rare.

What he is saying unless some one has grossly miscalculated with the original installation C type will be fine.

Of course one should measure loop impedance but if you consider the protection given with a 6A RCBO C type and a 6A MCB B type then the C type RCBO must be the way to go.

At around 32A with power not lighting then you reach the point where you may start considering using a B type. But unless one has made some mistakes with the installation under 32A it would be rare that a B type is required.

 

[bernardgreen]

Think I may just stick a 6a MCB in there until an RCBO pops up on ebay.

How will you ensure it is a working RCBO and not just a realistic looking counterfeit case with terminals and a bit of cast cast iron inside to make it weigh the same as a real RCBO

 

[JohnW2]

Think I may just stick a 6a MCB in there until an RCBO pops up on ebay.

How will you ensure it is a working RCBO and not just a realistic looking counterfeit case with terminals and a bit of cast cast iron inside to make it weigh the same as a real RCBO

I think that's much less of a concern with an RCD or RCBO than with an MCB. With an RCD/RCBO, there's a test button and the option to formally test (residual current) functionality - with an MCB, unless one x-rays it to give some reassurance that it seems to contain 'works', one has to rely mainly on prayer or hope! Admittedly, one has the same problem with the overcurrent protection part of an RCBO, but it's probably a lot less likely that a counterfeiter will fake only half of the device.

Kind Regards, John.

 

[JohnW2]

JohnW2 has given you the reason why at 6A the B type RCBO is rare. ... At around 32A with power not lighting then you reach the point where you may start considering using a B type. But unless one has made some mistakes with the installation under 32A it would be rare that a B type is required.

The figures/reasoning I presented apply equally to MCBs and RCBOs. If you think this is the reason why Type B RCBOs are 'rare', why are Type B MCBs anything but rare (indeed, generally the 'default')?

Kind Regards, John.