Clause 2.5.3.3

[ElectricalApprentice]

Evening all.

Just having a read of clause 2.5.3.3, alternative positioning of overload protection. Would an example of this be a 20A RCBO in a switchboard being feed from the main switch using 2.5mm cable? Even though the mains is fused at 63A, the 20A RCBO does not exceed the current carrying capacity of the 2.5mm conductor so it is permitted?

Thanks

[AlecK]

If you have mains proctected by 63 A HRC fuse, and a 2.5 mm2 from main switch feeding a single 20 A mcb;
then yes the 20A mcb can provide overload protection for the conductor feeding it as well as for the circuit it supplies.
Same if instead of a single 20 A mcb; it was 2 x 10 A mcbs. Or 4 x 6 A.

This rule also applies where we have a reduced-size mains active for a controlled or separately-metered load, between meterbox and MSB (eg for water heating).
The smaller cable can be protected against overload by the mcb for the w/h final subcircuit.

Protection of conductors against overload can be achieved by placing the device at either end, or anywhere along the run.
And can be achieved by a number of methods, not all of which involve use of any overcurrent device.


However overload is only part of overcurrent protection; we also have to protect all conductors against short circuit.
And that can only be done by using an overcurrent device at the upstream end

For both scenarios above the 2.5 won't have short circuit protection from a 63A HRC.
So we need either a bigger cable or different device.

[ElectricalApprentice]

Thanks AlecK. I always thought HRC fuses provided short circuit protection, why would it not is this case?

[AlecK]

They do, but as with all protective devices you have to select the protection required for the conductor in question.

The aim of s/c protection is to limit the time the high s/c current flows to less than the time needed for the particular conductor's temp to rise to the point where damage occurs
(typically by thermal damage to insulation in first instance; but ultimately melting the conductor itself if the current flows high enough for long enough).
The requirement is in 2.5.4.5; so just follow through the calculation.
Note that S (CSA of the conductor) is in the top line, so the bigger the cable the longer the time limit is for the protection to operate.
Conversely the smaller the cable the shorter the time limit,; and if you work it through you;l;l find that a 63 A HRC can't provide s/c protection for a 2.5 mm2. Generally OK for a 4 mm2; depending on the value of K in the particular circumstances.

Also read Section 5 of 3008.1.2 for further info on selection of conductors for short circuit performance.