Do we need a C or D rating on a RCBO?

[ericmark]

Do we need a C or D rating on a RCBO?

Not as daft as it my first seem. The magnetic part of the MCB ensures the MCB will trip in the required time in the event of a short circuit.

With the RCD being used any short circuit to earth with trip the device using the RCD part of the device.

So the B rating will only be required to ensure the device will trip if you get a short circuit line to neutral.

If one considerers a ring main of 106 meters of 2.5mm twin and earth (Max allowed for volt drop) fed from a TN-C-S supply so we can assume the incomer impedance line to neutral is 0.35Ω to trip a B32 trip will need 5 x 32 = 160 Amps so 230/160 = 1.4375Ω minus 0.35Ω that’s 1.0875Ω the twin and earth is doubled up so that’s 0.009Ω per meter which will mean it will trip at up to 241.6 meters.

So a B type RCBO will trip on the magnetic part of the device. But re-calculate that using a C type device and we drop to 81.94 meters and for a D type device 2.083 meters.

The original Dorman/Smith type LoadMaster MCB did not have any magnetic part and was a plain 5 to 70 amp MCB then we had the 1 to 4 type which was replaced with B to D type.

The maximum disconnections times given in the 17th Edition is for earth faults not line to neutral faults and any thermal trip will open before we get cable damage.

If we still need to consider the magnet tripping part of the trip will this place any limits on wiring which at the moment we are failing to consider?

Eric

 

[ban-all-sheds]

In TN systems it is preferable for RCBOs to operate in overcurrent mode when providing indirect shock protection. When they are working in overcurrent mode they are voltage independent, whereas in RCD mode they are not; they need a large enough voltage being applied to drive the circuitry inside. The IEC stipulate that voltage dependent RCDs should operate at voltages as low as 50V, so that the effects of a collapsing voltage in a fault condition are nullified. But if you get an open-circuit neutral, it is possible that an RCBO will not operate in RCD mode with a fault to earth, so the design for a TN system is supposed to ensure that RCBOs operate in overcurrent mode for indirect shock protection, and for this reason the earth fault loop impedence restrictions are the same, and a Type C or D RCBO may not provide a 0.4s disconnection time.

 

[Spark123]

Can I pinch that paragraph for the wiki please ban? The one on supplementary bonding for disconnection problems

 

[GaryMo]

In TN systems it is preferable for RCBOs to operate in overcurrent mode when providing indirect shock protection. When they are working in overcurrent mode they are voltage independent, whereas in RCD mode they are not; they need a large enough voltage being applied to drive the circuitry inside. The IEC stipulate that voltage dependent RCDs should operate at voltages as low as 50V, so that the effects of a collapsing voltage in a fault condition are nullified. But if you get an open-circuit neutral, it is possible that an RCBO will not operate in RCD mode with a fault to earth, so the design for a TN system is supposed to ensure that RCBOs operate in overcurrent mode for indirect shock protection, and for this reason the earth fault loop impedence restrictions are the same, and a Type C or D RCBO may not provide a 0.4s disconnection time.

Looks familiar. Paul Cooks commentary?

 

[ericmark]

The IEC stipulate that voltage dependent RCDs should operate at voltages as low as 50V, so that the effects of a collapsing voltage in a fault condition are nullified.

Thank you Ban-all-sheds very good point but the 50 volt surprised me as a very years ago we found some RCD's fitted to 110 volt supplies which all failed and when we tried to get 110 volt replacements we found most fail to operate at less than 100 volt and we had to locate special models that would work on a 110 volt system. Maybe all now changed. This I was told was why we used active types which fail safe. I note on Merlin Gerin they are listed as having a voltage range of 200 to 230 volt for the C60H RCBO under operating voltage. There were some with a published range of 110 to 240 volt but nothing as low as 50 volt.

I have seen many RCBO's where only option seems to be the C Type which is what prompted the question. In one section it tells us not to rely on RCD protection and next section saying we must use it. They are more reliable since we moved to current type but I feel only the active type can be relied upon to fail safe.

I have also seen this talk about the device taking the power from line neutral and neutral failure will stop them tripping except with active type of course but very little recommendation as where to use active and where to use passive types.

Note: Must not be used as the sole means of protection against direct contact with live parts (BS 7671) is printed on info sheet I know with Vigi you can select any type you want and in some makes Type B are available.

I am happy to use a RCD as an extra but not so happy when I know they will not trip on the magnet part of the trip although they will trip on thermal part.

 

[ban-all-sheds]

Can I pinch that paragraph for the wiki please ban? The one on supplementary bonding for disconnection problems

As GaryMo says it's from Paul Cook's commentary.

IANAL, but this rewording ought to make it OK:

In TN systems RCBOs should operate in overcurrent mode for fault protection. In overcurrent mode they are voltage independent, whereas in RCD mode they are not, and they obtain the voltage they need to supply the electronics inside from the supply circuit. The IEC stipulate that voltage dependent RCDs should operate at voltages as low as 50V, so that if the voltage collapses in a fault condition they will still work. But if the neutral is lost it is possible that an RCBO will not operate in RCD mode with an earth fault. So the design for a TN system is supposed to ensure that RCBOs operate in overcurrent mode for fault protection, and for this reason the earth fault loop impedence restrictions are the same as for an MCB, and a Type C or D RCBO may not provide a 0.4s disconnection time.

 

[ericmark]

Thank you BAS seems we should not use D RCBO and that we should be very selective using C type when using a TN supply then. Maybe someone should alert the manufactures about this?

 

[ban-all-sheds]

No different to the EFLI concerns when using C or D MCBs - why do the manufacturers need to be alerted? They make the products and tell you what spec they are - it's up to the designer/installer to decide if they are suitable...

 

[Spark123]

Cheers, have added it here:
//www.diynot.com/wiki/electrics:supplementary-bonding

 

[ericmark]

To BAS yes I agree in a perfect world that is what should happen. But you and I both know many firms send second year apprentice to wire houses and the poor guys have very little experience to draw on.

When the design of the house changes they don't have the skill required to re-design and just cross their fingers and follow their mates like a load of sheep.

I came across one site where the first few houses had special beams with knock outs for wires and pipes and electricians dream no drilling beams and the first few had ring mains using around 60 meters of cable.

Then the builder decided he could save money by using the normal wood beams which resulted in the 1/3 rule and the electrician just ran the cables on extended route. Result now needed 120 meters of cable not until third house with wooden beams was wired did the guy who did the inspecting and testing realise what was going on. Too late to alter those houses ended up with B25 amp instead of B32 amp MCB's to comply.

This happens all the time. How many electricians do a risk assessment and decide if it needs all RCBO's or twin RCD's. In the main very few! They have read that 17th Edition needs a twin RCD consumer unit and they just stick one in with little thought as to if it does comply with the regs. Its called a 17th Edition RCD so that's what they use.

Just because you and I put some thought into the job don't think everyone does. And if Merlin Gerin only make C32 RCBO's then that's what will get fitted and in the main most electricians just fit what they are given.

Not very proud of this but in my early years back in 1970's I could measure EFLI but I didn't have a clue if the results passed and I never measured the line / neutral impedance at all and there must be some work I did which was well out of speck. At that time there was no BS7671 and the 14th Edition would sit gathering dust in the managers office and even if the foreman was found reading it he got. "What do you want that for? just get on with the job". I would like to think that has all changed now! but I bet it still goes on. You see it on here where electricians from their answers have not read the book themselves but are just going on what they have been told.

So if you can go to the local whole sale outlet and buy a populated consumer unit with C32 RCBO's fitted they will get used by many electricians who will not even question if they are the correct ones for the job.

 

[Pensdown]

So if you can go to the local whole sale outlet and buy a populated consumer unit with C32 RCBO's fitted...

There's no chance of doing that in this part of the world. I've had some MK RCBO's on back order since October.

I also understand Wylex RCBO's are in short supply.

Maybe sparks have finally seen the light...or Johnd has sold his shares in pubic hair dyes and bought RCBO's instead

 

[ban-all-sheds]

I so don't want to know what you're talking about...