Testing and Inspection Over Current Protection

[John-]

Hi. Silly Question, as a tester / inspector of an installation. How do i confirm that overload protection is satisfied for an existing circuit, or is it just assumed as long as Coordination (p89 18th Regs) is satisfied and the protective devices is of a EN BS device? Practically, how do i confirm this though? Where do i find 'I2' details for OCPDs?

Thanks

John

 

[telectrix]

overload protection is provided by the MCB as long as Ib<In<Iz.

 

[John-]

overload protection is provided by the MCB as long as Ib<In<Iz.

Would that also apply to Fuses, like rewire-able ones as-well?

 

[pc1966]

Yes, fuses are following the same basic idea as MCB.

For rewirable fuses (BS3036) there is a larger margin between the fuse rating and the capacity of the wiring (see reg 433.1.202) but for enclosed fuses like BS88, etc, you treat them as for MCB ratings.

The bigger issue with rewirable fuses is if some muppet has used the wrong wire in the past, sometimes to "stop it blowing" without realising what danger it presents. Table 533.1 (p159 of 18th regs) has the nominal diameters of wire but you would need calipers or similar to check that in practice.

 

[Ian1981]

For 3036 fuses you apply the correction factor 0.725.
for example a cable Protected by a 15 amp 3036 fuse would need to carry 20 amps , if overload protection was required.
@pc1966 explains it nicely.