How does an RCD prevent a fire?

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240 Volt and 30 mA ( 0.03 Amp ) is 240 x 0.03 = 7.2 ( assuming my maths is correct )
Silly me of course, I used 230 volt and 300 mA so that's why did not match. My maths not very good either, odd can imagine numbers but calculus never could work it out, still use a slide rule, only to show off though, calculator much better. Just need to log that one up to experience?
 
In my opinion the RCD reduces the risk of a fire happening by limiting the amount of energy in the fault path to ground.
... it would, but only if the amount of current involved was not enough to cause an OPD to operate, anyway - and I would have thought that, in practice, any current high enough to result in a fire would usually (always?) be high enough to trip an OPD.

The circuit's OPD certainly should trip before current in the wiring of that circuit even exceeded the 'maximum operating temp' of the circuit's actual wiring, let alone a temperature of that wiring high enough to start a fire - so a risk of fire at a current that didn't trip the circuit's OPD could only arise if the current-carrying capacity of the fault path was much lower than that of the circuit's wiring - which I would have thought was pretty unlikely.

Kind Regards, John
 
But the idea of a 300 mA RCD or less to prevent fire still seems odd, ....
As I've said, although I may be missing something it sounds very odd to me. As I've said, I'm not convinced that a current lower than that which would trip a circuit's OPD (without van RCD) would be adequate to start a fire.
... although I know it works, I found a socket which had been damaged by water ingress that had part melted the casing behind the socket that had been made safe due to the 30 mA RCBO tripping, but signs of plastic distorting with just 30 mA or 6.9 watt, so what would happen to 300 mA or 69 watt?
For a start, that wasn't (and probably was nowhere near) a 'fire'. I have to say that I find it very hard to believe that 30 mA (or, indeed, 300 mA) flowing through anything in an electrical installation would cause any thermal damage, let alone a fire.

Of course, regardless of the IΔn of the RCD (30mA, 300mA or whatever), the device will not limit the current (only it's duration). However, the magnitude of the current is determined primarily by the resistance/impedance of the water, and I find it hard to believe that such a current (for any duration) would ever be enough to do thermal harm to anything.

Also, in terms of energy, remember that, in the case of water ingress, nearly all of the resistance/impedance in the fault path will be that in the water, so virtually all of the 6.9W or 69W you mention would be dissipated in the water, not in anything electrical.

Kind Regards, John
 
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and I would have thought that, in practice, any current high enough to result in a fire would usually (always?) be high enough to trip an OPD.

Think of a 5 watt lamp such as a Christmas tree lamp, that can ignite material in close contact with the lamp.

Then think of a thin wire as part of the fault path to ground
 
Think of a 5 watt lamp such as a Christmas tree lamp, that can ignite material in close contact with the lamp. .... Then think of a thin wire as part of the fault path to ground
I think that is getting close to 'barrel scraping':)

It's probably at least as likely that, due to a loose/poor connection, a joint in an L or N conductor is just 'hanging on' by one extremely thin thread/strand - but no RCD would help to detect that.

Kind Regards, John
 
I think they did some direct testing that established that a common source of earth faults is surface tracking on insulation and currents as low as 50-100mA have been found to be sufficient to cause ignition and fire.
 
I think they did some direct testing that established that a common source of earth faults is surface tracking on insulation and currents as low as 50-100mA have been found to be sufficient to cause ignition and fire.
I have to say that that surprises me but, if true, makes one wonder why the required only 300 mA RCD 'fire protection'(in most of the {seeming very few} situations in which such protection is required at all :)

Kind Regards, John
 
Not all adverse incidents ( accidents ) happen due to something at the top of the barrel.
Very true - such is the nature of the world.

However, despite that, we all get in/out of beds and baths, go up/down stairs, eat, use knives/tools etc. Some even go up/down ladders, travel in cars, cross roads etc., and yet another 'some' even engage in sports, drink alcohol or smoke etc.

Kind Regards, John
 

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