mcb - may have caused accident - alternative type available?

[Muddymike]

+1 We have three in the house.

Mike

[polygonum]

Recommended models? Links?

I did notice S Viemeister's Amazon link but there were few details nor any feedback.

[Bod]

Why not buy her a small lightweight LED torch that clips onto her clothing at all times? Quick, cheap and easy.

[polygonum]

Because I don't want a clipped-to-clothing LED torch. And I am not dave. :-)

[fred]

In case you don't already know, MCBs have both magnetic and thermal trips, magnetic takes care of short high transients and thermal handles longer duration but lighter overload trips.

[Bod]

[Martin Brown]

Trouble is some lamps particularly kitchen spotlamps fialing can draw a momenarily insane surge current that will take down the whole circuit.

A faster and safer alternative is to have a couple of emergency lights on standby so that if mains fails for whatever reason there is always some light in the kitchen and lounge (until the batteries run down).

These need not be massively expensive although the best ones with decent battery life tend to be a little industrial in appearence. It is a trade off between form and function.

A couple of cheap LED torches with a 500k resistor bridging the switch is also helpful if your mains is prone to dropping out.

Regards, Martin Brown

[John Rumm]

I use:

you chop them into a ceiling so that only the lens is proud they don't look quite as fugly.

[Andy Champ]

This has me curious. The main reason for the protection is so the wiring doesn't start a fire - the plug-top fuse is the one that protects anything delicate inside an appliance. So why the is fast necessary?

Andy

[alan]

Fit a non maintained battery backed up emergency light. Wire it in to the appropriate lighting circuit.

[John Rumm]

Its a good question, and the full answer is actually quite deep...

The reason protection is needed is actually two fold - as you say, fire is one issue. However shock protection is also another primary need. Over current protection operating in conjunction with earthing (and equipotential bonding) help provide both.

The reason for the two "speeds" of protection is the need to protect from two classes of over current, namely: "fault" and "overload". Over load has already been discussed - but refers to when a circuit is subjected to to a (typically) long term load in excess of its design current. Say for example a 32A socket circuit with too many appliances causing a total of 50A of load on it.

Fault current arises when you get a very low resistance connection (i.e. a "short") between line and earth or neutral. e.g. sticking a nail into a concealed cable, or cutting a flex with the mower. In these cases the magnitude of the current it limited only by the resistance of the wires in the circuit and those providing the supply. So the currents are

*very* high - typically hundreds, thousands, or sometimes even tens of thousands of amps.

With modern appliances, the fuse in the plug does not need to protect anything delicate inside the appliance, just offer "fault" protection to the appliance lead. (if an appliance needs additional protection, then it needs its own fuse - since in most locations outside of the UK there won't be a plug fuse).

The effects of these two classes of over current on wiring installations is very different, and hence they need handling in different ways:

Long term overloads will tend to cause excessive heating of the circuit wires - but don't necessarily cause immediate damage to the circuit in the short term. So for a limited period of time, the overload is acceptable. So the protection in the MCB reflects this current Vs time curve [1]. As you can see, relatively high overloads for short durations, or low overloads for longer times are permissible (and even desirable - it would be unwelcome if your socket circuit tripped every time you turned on the vacuum cleaner because its inrush current briefly took the circuit over its nominal maximum load for a couple of seconds).

For fault protection the problem is different. The currents involved here are likely to cause damage to a circuit very quickly - possibly even vaporising conductors, or even causing explosions. The thermal trip mechanics of a MCB would be far to slow to react in these situations. Hence there is a magnetic solenoid driven mechanism that reacts to very high overloads (5 times the rated maximum current in a normal type B device). Because the fault currents are potentially so high, circuit designers will often need to prove [2] that the conductor sizes selected can withstand these currents for long enough to give the MCB time to make the circuit safe before the conductors actually melt. This has particular relevance to earth conductors, where a failure of the conductor could leave users unprotected from shocks caused by live metalwork resulting from a fault.

[1]

[js.b1]

There are plug in night lights such as Osram Lunetta. When the lighting circuit trips out, these provide a floor level light.

Around 6-12 would be required for the floor level of a house, not cheap, but very cheap compared to the cost of a fall etc.

[meow2222]

go on the wiki somewhere

NT

[John Rumm]

> could go on the wiki somewhere

Yup, feel free, although not sure where it would go...

Most of it is there already - but spread around other articles.

The stuff on types of overcurrent is mostly in here:

rest is in the MCB article.

[Andy Champ]

So I won't quote it back. Thanks.

Andy

[meow2222]

No thank you

[ARW]

GU10 4.5W LEDs are beginning to be my option. And they certainly seem to be lasting these days.