HI guys got this of another website what do you make of it
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
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
