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Ever thought about ... ?

gkenyon
I was asked a series of interesting questions this week about fault protection and overload protection for a particular application. Some of these really make you think, and the physics doesn't always lead you where you think you'd go.


Dropping out of all this, was me pointing out something interesting which I wonder whether it's ever crossed the minds of contributors to this Forum ... so here goes.


Ever thought about what, in typical UK installations, protects the electronics in a plug-in [to a standard BS 1363-2 socket-outlet] phone charger / wall-wart type power converter against:

(a) Fault current (consider both cases of L-N and L-PE); and

(b) Overload current ?




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  • 0
    Indeed ... Sparkingchip and Davezawadi have worked out the real answer, in a round about way.


    In general, pluggable appliances are to assume a circuit protective device is located in the electrical installation, that may offer protection to the appliance connecting cord only.


    Fixed-wired product may rely on a specific type and/or rating of the protective device upstream in the installation ... but only where stated in the installation instructions (we've had this discussion in another thread).


    The difference in the UK being that we use a B32 and most other countries use either B16 or B20 for their socket-outlet circuits.


    My worry is really where the manufacturer of a wall-wart designed abroad assumes we have a 20 A protective device looking after the circuit the device is plugged into.


    So, I looked into this a bit more closely. Provided we are using a B32 mcb, the disconnection time is met by the overcurrent protective device (not an RCD, because of L-N faults), and the fault current does not (as DaveZ has said is unlikely) exceed 3 kA at the socket-outlet, there is really no problem. This mirrors what we all know - for most direct shorts on appliances or appliance flexes, a B32 will operate before 13 A and 5 A fuses ... and sometimes 3 A fuses.

    HOWEVER this is NOT the case for BS1361, BS88, or BS 3036 rewireable fuses, with BS 3036 being perhaps the worst-case let-through.


    My conclusion is that things are probably OK for the conditions in most installations with B20 and B32 mcb's, but perhaps not OK in installations with rewireable fuses, where the use of an extension lead (to provide BS 1362 fusing at 13 A) is probably recommended.
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  • 0
    Indeed ... Sparkingchip and Davezawadi have worked out the real answer, in a round about way.


    In general, pluggable appliances are to assume a circuit protective device is located in the electrical installation, that may offer protection to the appliance connecting cord only.


    Fixed-wired product may rely on a specific type and/or rating of the protective device upstream in the installation ... but only where stated in the installation instructions (we've had this discussion in another thread).


    The difference in the UK being that we use a B32 and most other countries use either B16 or B20 for their socket-outlet circuits.


    My worry is really where the manufacturer of a wall-wart designed abroad assumes we have a 20 A protective device looking after the circuit the device is plugged into.


    So, I looked into this a bit more closely. Provided we are using a B32 mcb, the disconnection time is met by the overcurrent protective device (not an RCD, because of L-N faults), and the fault current does not (as DaveZ has said is unlikely) exceed 3 kA at the socket-outlet, there is really no problem. This mirrors what we all know - for most direct shorts on appliances or appliance flexes, a B32 will operate before 13 A and 5 A fuses ... and sometimes 3 A fuses.

    HOWEVER this is NOT the case for BS1361, BS88, or BS 3036 rewireable fuses, with BS 3036 being perhaps the worst-case let-through.


    My conclusion is that things are probably OK for the conditions in most installations with B20 and B32 mcb's, but perhaps not OK in installations with rewireable fuses, where the use of an extension lead (to provide BS 1362 fusing at 13 A) is probably recommended.
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