Fused spur, for human or wiring safety?

[epsilonGreedy]

I was ordering the bits for my shed wiring mini project and the knowledgeable bloke behind the counter said "and of course you will be wanting a 5amp fused switch as you step down from 2.5 mm2 cable to 1.5 mm2 for the lighting branch". I frowned and he clarified "to protect the wiring".

 

During the drive home I was pondering this advice and concluded RCDs protect humans and fuses protect cables, have I got this right?

[Jeremy Harris]

5 minutes ago, epsilonGreedy said:

I was ordering the bits for my shed wiring mini project and the knowledgeable bloke behind the counter said "and of course you will be wanting a 5amp fused switch as you step down from 2.5 mm2 cable to 1.5 mm2 for the lighting branch". I frowned and he clarified "to protect the wiring".

 

During the drive home I was pondering this advice and concluded RCDs protect humans and fuses protect cables, have I got this right?

 

The bloke's right.  You need to either fit a fuse or fit a suitably rated MCB to protect the 1.5mm² lighting circuit, as the 2.5mm² (assuming it's a radial) will have a higher rated overload protective device.

 

Often it makes sense to fit a "garage CU", so that the circuits in the shed can be split into a power circuit (with a 16 A or 20 A MCB) and a lighting circuit (usually with a 6 A MCB).

 

[Ferdinand]

6 minutes ago, JSHarris said:

You need to either fit a fuse or fit a suitably rated MCB to protect the 1.5mm² lighting circuit, as the 2.5mm² (assuming it's a radial) will have a higher rated overload protective device.

 

If you leave the 1.5mm² 'protected' by the larger fuse, then it can lead for example to your smaller wire burning out before the fuse itself - which (in addition to allowing a bigger current through, fire hazard etc) is a far more complex thing to mend afterwards.

 

Apols if adding eggs to your aniseed balls to suck.

 

F 

Edited May 21, 2019 by Ferdinand

[ProDave]

A lesson in what happens if an untrained person does some wiring, and the bloke in the shop does not advise him.

 

I have lost count of how many times I have found inappropriate cables and loads just spurred off a 32A ring final with not other protection. 

[epsilonGreedy]

3 minutes ago, JSHarris said:

Often it makes sense to fit a "garage CU", so that the circuits in the shed can be split into a power circuit (with a 16 A or 20 A MCB) and a lighting circuit (usually with a 6 A MCB).

 

 

Had it been a long term shed installation I would have followed your advice but planning will demand the removal of the shed within a year. I did the job as a learning experience, the bloke behind the counter even persuaded me to use earth coloured insulator for the exposed T&E earth. I must confess I did not colour code the live return from the light switch down the blue wire.

 

I found the sizing the supplementary earth insulator such a hassle I have already decided for the main house project I will buy proper Brown & Brown T&E for the light switch branches.

[ProDave]

If you loop at the light switches, no need for that.  Brown and blue into the switch, brown and blue out of the switch.  simple.

[epsilonGreedy]

13 minutes ago, ProDave said:

A lesson in what happens if an untrained person does some wiring, and the bloke in the shop does not advise him.

 

 

Point taken, perhaps I have too much faith in the magic powers of RCDs. If anything abnormal happened in the low amp branch and 32amps worth of electrons tried to charge down a shorting 1.5mm cable, would the RCD not trigger faster than even a correctly sized fused spur?

[epsilonGreedy]

18 minutes ago, Ferdinand said:

If you leave the 1.5mm² 'protected' by the larger fuse, then it can lead for example to your smaller wire burning out before the fuse itself - which (in addition to allowing a bigger current through, fire hazard etc) is a far more complex thing to mend afterwards.

 

 

Ah so there is an economic argument in addition to safety, I will remember that.

[ProDave]

1 minute ago, epsilonGreedy said:

 

Point taken, perhaps I have too much faith in the magic powers of RCDs. If anything abnormal happened in the low amp branch and 32amps worth of electrons tried to charge down a shorting 1.5mm cable, would the RCD not trigger faster than even a correctly sized fused spur?

If the short is L to N the RCD will not trip. At least not until the cable has melted.