RCD tripping with mains saw

[John Rumm]

Indeed that may or may not be safe depending on the external earth impedance at the origin, since that MCB will need 320A to open on the instant part of the trip curve.

If you had the notional book value of 0.8 Ohms of external earth impedance on a TN-S earthing system, and added to that the 42 mOhms contribution of the cable, you would only be able to muster about 273A of fault current. (which would take something like 7 seconds to open that MCB).

An adiabatic check would also show the conductor size required:

s = sqrt( 273^2 x 7 ) / 115 = 6.28mm^2

so well in excess of the 1.5mm^2 area of earth conductor.

If we work it backwards from the 320A required, the total ELI can't be more than 230 / 320 = ~0.7 ohms. So fine without measurements on a TN-C-S system, but will need checking on TN-S

[Roger Hayter]

Sorry I missed we'd changed from type B to type C. That makes a dfference I suppose? With my limited knowledge I think I just know that ordinary circuits will be all right with 20A type B and if I want anything more unusual I'll ask.

[Jack Lawson]

Please see earlier posts - it is not a ring - it is a very short spur direct in to a dedicated MCB.

[John Rumm]

The C type has the same overload protection (i.e the thermal part of its response is the the same) as the type B, but its magnetic response is less sensitive. A type B will open "instantly" on fault currents of 5 times the nominal current, so 160A for a B32. The C type devices require

10 times the nominal current to open instantly, or 320A for a C32

The effect is to make them far more tolerant of high inrush currents, but at the expense of possibly lowering the protection for fault currents, so in some cases one will need to check (by measurement or calculation) if a C type is safe to use on a particular circuit.