Fusing theory

And the answer is simple. We use a ring main, rated at 32A to drive sockets rated at 13A max.

The rest of the world uses switched spurs.

Not 2 phases. One centre tapped phase.

Because they have use relatively low current circuit breakers in the CU on the understanding that the circuit breaker will have to protect not just the circuit cables from fault currents, but also the (potentially much smaller) appliance flexes.

This leads to needing lots of small circuits to provide adequate numbers of sockets, and the extra inconvenience for the users since it is not difficult to find a selection of appliances that will trip the MCB.

Here with a 32A circuit, we can festoon the place with sockets (100m^2 worth on a ring) knowing it is quite hard to conjure up enough load to trip that on overload, and that the plug fuses will offer fault current protection to the appliance flexes.

See my reply elsewhere in this thread about adiabatic calculations.

In practice there will be a far more fragile link in the device itself. IN the case of a filament lamp, the filament

Indeed.

Remember fuses don't limit current (they have no mechanism to do so), all they can do is limit the duration of fault conditions.

So a fault current can be 1000A and the fitted fuse will not change that. What the correctly sized fuse will ensure is that the 1000A is not allowed to flow for long enough to cause damage.

And a lot of LED lamps have a low value resistor (1 or 2 ohm) as the first component in their internal PSU to act as a fuse (and an inrush current limiter).

I always understood that the fuse was there to protect the house wiring i.e. the ring main, so that if a short develops in the flex coming from the socket to the table lamp or whatever (the dog chews through it, for example) then the ring main doesn't end up overloaded, overheats, catches fire and burns the house down. Never mind what happens to the dog!

With respect, that's nonsense!

MCBs don't limit fault currents (or any other current) - only ohms can limit current.

It is one of the reasons that many appliances have short cables. It means the maker can cheap out on thin flex, and still stay just inside the threshold where the flex will survive the anticipated fault current for long enough to operate the circuit protective device.

Or just maths :-)

Na, generally that is what the MCB / Fuse / RCD in the CU is for.

(although there are cases where you can delegate overload protection downstream, fault protection must always be upstream)

The video describes it as two phases: +120V / -120V

They interrupt the current if you draw too much, ie the fault current is time-limited. If you draw very too much, the time is limited even more. The damage done by a fault current is usually due to heating and that's a function of both current and time. So the MCB limits the integral of current over time, which is what causes damage, if not the instantaneous peak current. Indeed, many appliances wouldn't work if they did trip based on an instantaneous peak current.

(I'm glossing over the fact that a 16A MCB won't trip at 16.01A even continuous, but it's highly likely that a fault current will be substantially higher than the rated current so this won't matter)

That's notable on power tools - and of course people put them on extension leads.

Theo

Indeed - that is the key, the MCB can only limit duration not magnitude.

Once you hit the fault current threshold, the response time will be pretty much the same regardless of magnitude. Fuses get faster with higher fault currents.

Yup the so called "let through" energy - I^2 x t

Hence why the fault trip kicks in at 5x, 10x, or 20x nominal current on a type B, C, or D MCB

Power tool leads are not usually that skinny IME although some do have too short leads (or too stiff). 4m is a decent length I find.

You can check the adiabatic current if you want. But there’s not much you can do about the current that is drawn in a fault condition....

Says he who just knocked out a substation in Leeds:-)

It probably needed new fuses anyway so it keeps people in a job.

So have you got copper-coated eyebrows?

Nice :-)

How did that happen?

-- Cheers,

John.

/=================================================================\ | Internode Ltd -

Hmm.. I had just *reluctantly* taken over as Chief Electrical Engineer when building contractors put a digger bucket through the factory 11kV ring main. Normally run open to limit fault current.

The senior electrical foreman was confident the fault could be overcome by re-routing the supplies and closed the ring into the fault! This shut down the entire factory!

During substation maintenance next Summer we discovered the 11kV switch contacts and the 132kV incomer contacts needed replacement!

The contractors insurers got a significant bill as they had been told to dig by hand.

Did this involve you being sarcastic to an apprentice along the lines of "Yeah, it is OK to put a metal bar across L & N before the cutout fuse as a safe means of isolating a house before working on the CU"?

And said Apprentice went and put a tyre lever across said cutout?

Its one way of getting rid of the apprentices you don't think much of I guess...... :-D

As an aside, I found an old pack of plug fuses marked 240V 50Hz. Does the fuse relate to the frequency? I assumed as a thermal device it would not. Also, do any 60Hz countries use BS fuses anyway?

Worse than LN.

It was LL.

Something dropped down a badly designed/installed incoming main isolator.

Not sure what the something was as it melted.

Anyway it was an expensive repair as it involved digging up the road to replace the isolator.