When to fit type C RCBOs?

[sjwkmje]

Hi All,

I am fault finding some one elses work today.

Had some nusance tripping on a supply to a workshop, and a ring main in the workshop. Everything tests out ok. The only time I could get it to trip was when a welder was plugged in (16A BS4343), and a big isolation transformer was powered up supplying a bench for electronic testing. The isolation transformer is on a ring final. Both circuits are fed using RCBOs with an RCBO upstream. So we have a type b 16a for the ceeform and a type b 32a for the ring.

The supply is fed via a type b RCBO upsream. I need to work out the discrimination here. I think this upstream RCBO should be type s so that the downstream devices trip in a fault not the upstream devices. I am not even sure why there is an RCBO feeding the workshop. The run is in 10mm SWA to 16mm t + e inside with the gland bolted into a metal enclosure, a din rail terminal setup and the t+e entering via a stuffing gland. Nice job all IP66 by the looks of it. With a seperate g/y 10mm fed to this metal enclosure. The enclosure is earthed via the SWA gland and a earth block inside connects the t+e earth, a flying lead to the box, and the 10mm g/y fed to this. The t+e is feed from a crabtree starbreaker 10 way board, a submain from split meter tails. This board has a mix of RCBOs of the correct type and MCBs.

Looking over my results I don't think an RCBO is needed upstream. Just add local protection inside the workshop. Its just going to lead to a discrimination headache.

Why would there be an RCBO on the feed? I cannot think of anything and had a look in the red book to see if I over looked anything today and the OSG.

My plan, replace the type B RCBOs with type C to feed the ring and 16A BS4343. Its a domestic setup and I have never fitted any type c breakers in a domestic install, only industrial where there are loads with large inrush. In on a type c is 5 to 10 and type c is usally used for inductive loads. Where In on a type b is 3 to 5. So ideal for this situation.

Would this be deemed ok?

Next problem the exterior lighting circuit is on a type C 10A RCBO. I have no idea what the spark was thinking. There is a SOX lamp and lots of fluros that's about it. Why is there a type C there? I can understand the thinking of an RCBO.

The installation is really nice but there seems to be allot of RCBOs and no discrimination.

What are you thoughs?

Adam

 

[festa]

Type C breakers are used for high starting/inrush current`s or like me,if the supplier has run out of type B`s getting hold of type B RCBO`s is a mare at the moment.
The only real problem is the allowable max Zs`s of these type C`s
Example...
Type B 32A BS7671 states a max Zs of 1.44ohms
Type C 32A ..BS7671 states a max Zs of 0.72 ohms.

 

[sjwkmje]

Type C breakers are used for high starting/inrush current`s or like me,if the supplier has run out of type B`s getting hold of type B RCBO`s is a mare at the moment.
The only real problem is the allowable max Zs`s of these type C`s
Example...
Type B 32A BS7671 states a max Zs of 1.44ohms
Type C 32A ..BS7671 states a max Zs of 0.72 ohms.

I forgot about Zs. The upstream 40A RCBO would probally be within spec to be changed to a type c as there is a parallel earth path as there is a 10mm g/y. That does not solve the discrimination problem.

The ring final Zs would be fine for a type c as well so I will change that out as well.

The upstream RCBO I think that needs to be swapped out for a type s or a MCB. Don't see any need for it been a RCBO. Problem is can you get a type s single pole crabtree starbreaker RCBO

Thanks,
Adam

 

[ban-all-sheds]

Its a domestic setup and I have never fitted any type c breakers in a domestic install

I'm minded to use C6s for my lighting circuits...

 

[RF Lighting]

Ditch the RCBO from the feed.

It is not needed, and bad installation design.

Swap the RCBOs for the final circuits in the workshop for type C. Some welders will require a type D, but this requires a maximum Zs of 0.29 ohms for a 32A OCPD.

Leave the 10A type C in the house alone. The inrush to fluorescents can trip a type B. Thats probably what the spark was thinking.

 

[sjwkmje]

Ditch the RCBO from the feed.

It is not needed, and bad installation design.

Swap the RCBOs for the final circuits in the workshop for type C. Some welders will require a type D, but this requires a maximum Zs of 0.29 ohms for a 32A OCPD.

Leave the 10A type C in the house alone. The inrush to fluorescents can trip a type B. Thats probably what the spark was thinking.

I will leave the 10A type c in place. Zs is within tolerance to do this.

I will remove the 40A type c rcbo and change it to a MCB type c. I considered the RCBO to be a bad design just wanted to make sure I did not overlook something.

Thanks for the advice,

Adam

 

[plugwash]

I see all this talk of ditching the RCBO feeding the submain yet I see no information here on what cable type is being used for the submain and whether there is someone appropriate and prepared to become a skilled or instructed person monitoring this installation....................................

 

[sjwkmje]

I see all this talk of ditching the RCBO feeding the submain yet I see no information here on what cable type is being used for the submain and whether there is someone appropriate and prepared to become a skilled or instructed person monitoring this installation....................................

You mean the cable supplying the CU in the garage from the upstream RCBO in the CU in the house. The run is in 10mm SWA to 16mm t + e inside with the gland bolted into a metal enclosure, with a seperate g/y 10mm fed to this metal enclosure.

The supply to the CU in the house (there are two but for the purposes here the CU in the house is the only one I am working on) is from 25mm2 tails. With a 16mm^2 g/y to a earth block.

Plugwash the installation would be better without the upstream RCBO IMHO as there will be no nusance tripping.

Adam

 

[plugwash]

The run is in 10mm SWA to 16mm t + e inside with the gland bolted into a metal enclosure, with a seperate g/y 10mm fed to this metal enclosure.

is the 16mm T&E a concealed cable at any point along it's run?

Plugwash the installation would be better without the upstream RCBO IMHO as there will be no nusance tripping.

I would generally agree but given the new rules on RCDs in the 17th edition it is nessacery to check that the submain is compliant without a RCD before removing the RCD protection from it.

 

[sjwkmje]

The run is in 10mm SWA to 16mm t + e inside with the gland bolted into a metal enclosure, with a seperate g/y 10mm fed to this metal enclosure.

is the 16mm T&E a concealed cable at any point along it's run?

Plugwash the installation would be better without the upstream RCBO IMHO as there will be no nusance tripping.

I would generally agree but given the new rules on RCDs in the 17th edition it is nessacery to check that the submain is compliant without a RCD before removing the RCD protection from it.

The submain CU is in a space behind the immersion heater, there is a partition wall and a door into one of the upstairs bedrooms. The tails drop into the floor across and down in a channel to the meter cuboard. Run is about 10M with a switch fuse (60A).

The 16mm t+e runs from this CU into the floor for about 1M not surrounded by anything its in open space, then up inside a cavity wall with no insulation for about 40mm then out into YT4 into the loft. Runs across the loft cable tied using cable tie bases to the enclosure where the SWA enters from outside. The SWA the runs down and across the house to a trench.

No where does the t+e get burried in insulation. I do not have the red book on me but I do not think the t+e needs to be derated at all.

I understand the implementations of the 17th edition.

 

[hairyben]

RCBO's are single pole. evan if you had downstream discrimination eg time delay between them a N-E fault could take out both in line, and continue to trip the submain feed with the circuit RCBO switched off. The only way round it would be to use DP RCBO's in the shed CU.

The other thing to watch is if you're going for c-types in the shed, be good practice to have a c-type on the submain too, lest it become the weakest link.

C6/10 and B10 mcb's are far more likely to trash dimmerswitches when a lamp fails.

 

[GaryMo]

RCBO's are single pole.

Not always - Just one example

Never come across one myself though

 

[Spark123]

Have seen SPSN (single pole switch neutral) varieties in MK, Merlin Gerin and GE before, think Hager make them too. They are usually double module width.

edited - typo

 

[hairyben]

Yes I could have perhaps phrased that better such as most typical RCBO's in general use will be SP, but I did mention & recommend DP RCBO's for the downstream board though.

 

[sjwkmje]

Have seen SPSN (single pole switch neutral) varieties in MK, Merlin Gerin and GE before, think Hager make them too. They are usually double module width.

I have a hager 63A type b somewhere in the junk in the garage taken off a job in work.

RCBO's are single pole. evan if you had downstream discrimination eg time delay between them a N-E fault could take out both in line, and continue to trip the submain feed with the circuit RCBO switched off. The only way round it would be to use DP RCBO's in the shed CU.

The other thing to watch is if you're going for c-types in the shed, be good practice to have a c-type on the submain too, lest it become the weakest link.

C6/10 and B10 mcb's are far more likely to trash dimmerswitches when a lamp fails.

The only way to do this correctly would be to split the tails, fit a type s double pole RCBO to feed the 16mm t+e then the other end a main switch in the CU with type c RCBO's to protect the required circuits. This is how I would have installed it if I had designed it.

In this situation I was considering MCB from the sub main cu to a double pole type s RCBO in a second enclosure feeding the t+e but this would be bad design.

I think I will change the upstream RCBO for a type c MCB instead.

I was asked about the possibility of adding a isolator to isolate the supply in the house. Now 50A, there is not many isolators that look good in a domestic setting that can take 50A. I can think of a few industrial style switch disconnectors. I am sure, still looking in the red book that a MCB cannot be used as a functional switch, but I also need to isolate netrual so require double pole.

Looking in the RS cat I found 282-0856. You know you have been working as an industrial spark for to long when you use crimps on everything in a domestic setting even earth lugs on KO boxs, use ferrules on flex for connections to FCUs and suggest fitting one of those! I even considered a contactor with a FCU for the coil supply off the ringmain in a metal enclosure and panel mount isolator on the front.

Adam