If an appliance is plugged into an extension lead, and the lead is plugged into a socket outlet - will a fault in the appliance blow (a) the fuse in its own lead, (b) the fuse in the extension lead or (c) both?
Is there a general rule of thumb?
The MCB isn't tripping.
(trying to fault find daughters washing machine over the phone, can't get round till tomorrow pm).
The most inaccessible will blow - Everytime! :)
Silly question - I assume the fuses fitted in the plug and the extension lead the same rating?
The current drawn through them is the same so the 'weakest' of the two will blow.
Are you sure the extension lead can handle the current? Is it unwound fully? If partly coiled it will get dangerously hot. Do you know the current rating of the washer?
The Medway Handyman presented the following explanation :
If both are the same value of fuse, either or both could blow - assuming the washing machine is the only load on the extension. Normally an MCB would be the first to trip.
In message , The Medway Handyman writes
Weeeeeeeellllll....
You would think that the appliance fuse (given that they were the same type of fuse with the same rating) would go first, but there is no hard and fast rule that can be applied
I've had cases where e.g. a 2A fuse in a circuit has survived when a
4A one (of the same type) has wimped out firstFuses can be complicated little beasties - they blow according to I^2xt (current squared x time it is applied for) in an ideal world
Alas all 13 amp :-(
Lead is only a 1 metre 3 gang, fully 'unwound' - not that you could wind it really.
Tumble drier & small dishwasher are on the same lead, their fuses seem to survive.
Latest reports (via phone) are that there is a 'flash & bang' from 'somewhere near bottom of washing machine'.
Bear in mind this is a daughter who calls me to ask me to make a shelf 'three Argos catalogues long'.
No not really. Cascaded fuses of the same rating won't be able to discriminate. Either could go, or possibly even both (the latter is probably only likely on a hard short circuit rather than a longer duration overload)
I don't have trip characteristics for BS1362 (i.e. plug fuses) to hand, but if you look at a BS1361 15A cartridge fuse (the type used in fuse carries in some CUs), you can get a feel for the current required to open the fuse in the 0.1 sec time frame - and that is going to be around
90A. A 32A type B MCB however will need 160A to open instantly on the magnetic part of its trip curve. So if you are a reasonable distance from the CU, and especially with a few extra connections about due to the use of the extension lead, there is a fair chance there is enough resistance in the circuit to limit the fault current to non MCB tripping levels.Ah, lucky you!
Eeek! You really mean that there is what could easily be more than 6kW of load on one poor little 3kW extension lead of questionable quality?
Short on the mains input filter would be a good possibility - if it goes the moment power is applied.
Ah, so tell her she probably has an Ikea and a Matalan catalogue too many plugged into her lead then!
9kW (TD + DW + WM)
This is exactly the sort of scenario that Part P was explicitly predicted to encourage.
Owain
No you would not. There is no reason to expect the appliance fuse to fail before the extension's fuse.
I was being a little conservative since he did say "small" dishwasher, but yup, it could easily be 9kW.
Indeed, and it does.
If you look at fuse rating curves, you quickly notice 2 things a) what large currents are needed to pop them b) how widely their characteristics tend to vary
in short, there's no way to know which 13A fuse will pop. However if more than one appliance were on at once, the fuse seeing highest current would be more likely to pop first, ie the extension lead fuse.
9kw on a 13A lead - fun! Hope shes got a camera, smoke alarms and a designated fire escape route.As it sounds like a dead short I cant see how she'd solve it, surely a WM can wait till tomorrow?
NT
to go and the circuit MCB.
If the fault current is relatively low it will just take out the weakest fuse.
Current can still flow through a molten wire or plasma of metal vapour. I have known glass fuses to shatter, but not the standard mains ceramic, sand filled types.
In message , "dennis@home" writes
He obviously thought that one of them would have gone first - that's why he asked, most people think that the one nearest the fault would go first. TMH is not someone with the depth of knowledge in such things - again, which is why he asked
d*****ad
Hadn't thought about the overload, good point. Have warned daughter not to use more than one appliance at a time.
The plot thickens! Didn't get round tonight, but gleaned over the phone;
There is one two gang socket outlet in the utility area. One outlet supplies a fridge/freezer, the other oulet supplies the three gang extension lead.
The dishwasher (table top model) sits on top of the washing machine, the two gang socket is mounted on the wall level with the top of the WM. The dishwasher has apparently been leaking.
Got daughters 'bloke' to turn power off, remove socket cover & spray with WD40 to make sure the socket wasn't damp/wet. There is only one 2.5mm cable, so I assume this is a spur.
Next on the list is to get the washing machine checked, but long term she can't keep having that kind of load on a three gang extension plugged into a spur.
What sort of load will a spur cope with if I replaced the two gang with a four gang?
Not much utility in that utility area then!
That will help - not.
could be - not worth assuming at these sorts of loads though.
A spur with one double socket on it has a design load of approx 20A - allowing for a bit of diversity. Its worth using decent quality accessories under these circumstances as well.
If you swap to a 4 gang socket, you will *reduce* the total capacity (nominally at least) to 13A since the socket will be fused (since you can't have 4 sockets on an unfused spur). Note however she is already limited to 13A via the extension lead - so not actually that different apart from the fridge not sharing the 13A fuse. The fridge itself is not going to be a big consumer, however its inrush current as the motor starts may be quite high.
Really in this case you need to find where the spur cable comes from and see if you can take a second cable there. Then stick in a extra pair of double sockets and join them into the ring.
A single 2.5 mm^2 cable is good for about 20 amps - and will thus support a
*total* load not exceeding about 4.6kW
Thanks John
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