As an interested DIY enthusiast I watch these forums a lot and occasionally post; I've never done anything electrical and leave that to the professionals when needed. I'm going to have a new CU fitted soon and I have learnt a lot from reading the posts here. I understand most of it but one thing intrigues me. I know that the main purpose of an RCD is to detect earth leakage and prevent a shock if someone comes into contact with the rogue current, but how does this prevent a fire? Is it because the leakage can cause a spark?
How does an RCD prevent a fire?
- Thread starter Vicario
- Start date
Sponsored Links
If the current leaks to earth through a person (i.e. they get a shock) the current will be pretty low (albeit high enough to do harm, or even kill, a human being) and an RCD is, indeed, intended to detect that situation and disconnect the supply to limit the duration of the electric shock.
However, if a leakage to earth arises due to some fault (in wiring or some appliance etc.), that leakage current can be much higher, potentially high enough to overheat wiring and hence start a fire. In most electrical installations, such a fault should be dealt with by other devices (fuses or 'breakers') but an RCD gives added protection - and, in some installations may be the only protection against such eventualities.
Kind Regards, John
- Joined
- 27 Jan 2008
- Messages
- 23,630
- Reaction score
- 2,661
- Location
- Llanfair Caereinion, Nr Welshpool
- Country
Good question, a 300 mA RCD is used as fire protection, but to be frank not sure of the thinking behind this.
Is that not only if there is inadequate fault protection provided by ADS? (i.e. my "... in some installations may be the only protection against such eventualities")
Kind Regards, John
Sponsored Links
Reg 532.2
RCD's not exceeding 300ma can be used for fire protection.
Chapter 42, gives you the scope behind it.
Its not just fire, but the effects of Heat or fire, including Heat and smoke
RCD's not exceeding 300ma can be used for fire protection.
Chapter 42, gives you the scope behind it.
Its not just fire, but the effects of Heat or fire, including Heat and smoke
- Joined
- 27 Jan 2008
- Messages
- 23,630
- Reaction score
- 2,661
- Location
- Llanfair Caereinion, Nr Welshpool
- Country
So where is it necessary to limitBS7671:2008 said:
consequence of fault currents in a wiring system from the point of view of fire risk?
So it seems for under floor heating etc, however never really worked out when 30 mA and when 300 mA the CONSTRUCTION AND DEMOLITION SITE INSTALLATIONS also had 500 mA.BS7671:2008 said:
Where we got all the 100 mA from I don't know? It seemed to be the standard size at one time, and of the S type
the idea was to be a balance between protection and nuisance tripping and it seems for fire protection.Western Automation R&D said:
I think what you may be missing is that ....
... appears only applicable when the requirements of 422.3.9 apply and, being part of 422.3, that regulation appears to relate only to ...
I'm not sure what "processed or stored materials" we are talking about, but it seems that the requirements of 532.1 only apply in very special circumstances, probably not ones which exist in domestic, or many other, premises.
Having said that, I'm not at all sure, even in those 'very special circumstances', what the RCD is meant to achieve if the circuit already has adequate fault protection.
I've often wondered where the 100mA came from (300mA would presumably usually serve the purpose). I have up-front 100 mA type S RCDs, but they are to provide the required fault protection to the lengthy distribution circuits in my (TT) installation - not anything specifically to do with fire.
Kind Regards, John
See my response to eric. I have a feeling that regs which relate to only very special and unusual situations are being pulled out and treated as if they apply generally.
Kind Regards, John
Germans have been using Type B+ RCD for awhile and they are designed to protect against fires caused by high frequency ground fault current.
The frequency tripping curve of the Type B+ is more sensitive to high frequency capacitive earth leakage current, so these devices are not recommended for use when circuits are equipped with switching mode converters or in situations where unwanted tripping needs to be avoided.
The high frequency leakage current results from either stray or EMC filter capacitors under normal working conditions. Capacitive leakage currents are not dangerous and cannot generate any electrical fire. In the case of a ground fault, the earth leakage current will not be of a single frequency but will be composed of different frequencies(mainly network frequencies and high frequencies linked to switching mode equipment in the circuit). Such a composite fault scenario is interpreted as a Type F RCD by the IEC 62423 standard. Since standard Type B RCDs complies with the requirements of the Type F RCD, the ground fault situation for fire protection is generally covered without having to resort to a Type B+ RCD.
I believe the limits were set with actual testing?
The frequency tripping curve of the Type B+ is more sensitive to high frequency capacitive earth leakage current, so these devices are not recommended for use when circuits are equipped with switching mode converters or in situations where unwanted tripping needs to be avoided.
The high frequency leakage current results from either stray or EMC filter capacitors under normal working conditions. Capacitive leakage currents are not dangerous and cannot generate any electrical fire. In the case of a ground fault, the earth leakage current will not be of a single frequency but will be composed of different frequencies(mainly network frequencies and high frequencies linked to switching mode equipment in the circuit). Such a composite fault scenario is interpreted as a Type F RCD by the IEC 62423 standard. Since standard Type B RCDs complies with the requirements of the Type F RCD, the ground fault situation for fire protection is generally covered without having to resort to a Type B+ RCD.
I believe the limits were set with actual testing?
Last edited:
Interesting. I have to say that "fires caused by high frequency ground fault current" is a new one on me - do the Germans specialise in such things?
Kind Regards, John
In my opinion the RCD reduces the risk of a fire happening by limiting the amount of energy in the fault path to ground. The maximum heat that can be produced in the fault path to ground in Watt is 240 V x 0.03 A = 7.2 W for a very short duration ( operating time of the RCD )
If that 7.2 Watts was not time limited and was concentrated in a very short length of thin wire then that length of wire could become hot enough to ignite flammable material in contact with the hot wire. This could then result in a fire being started.
An 8 Watt 230 volt filament lamp connected between Live and Earth will cause the RCD to trip before the filament can become hot enough to emit any light.
A 100 mA RCD will limit the heat to 24 Watts. Still not enough energy to create an ignition in any normal installation where the heating from the energy in the fault path to ground will be spread over several inches if not feet of conductive path.
If that 7.2 Watts was not time limited and was concentrated in a very short length of thin wire then that length of wire could become hot enough to ignite flammable material in contact with the hot wire. This could then result in a fire being started.
An 8 Watt 230 volt filament lamp connected between Live and Earth will cause the RCD to trip before the filament can become hot enough to emit any light.
A 100 mA RCD will limit the heat to 24 Watts. Still not enough energy to create an ignition in any normal installation where the heating from the energy in the fault path to ground will be spread over several inches if not feet of conductive path.
RCDs do not limit the magnitude of the fault current, only its duration.
I should have said the RCD limits the duration of any heating greater than 7.2 Watts that occurs in the fault path to ground.
The duration of time during which more than 7.2 Watts is generated is ( normally ) too short for an excessive rise in temperature to occur
The duration of time during which more than 7.2 Watts is generated is ( normally ) too short for an excessive rise in temperature to occur
- Joined
- 27 Jan 2008
- Messages
- 23,630
- Reaction score
- 2,661
- Location
- Llanfair Caereinion, Nr Welshpool
- Country
I think you are likely correct 69 watts (230 x 0.3) will not generate much heat, not sure where the 7.2 comes from?
But the idea of a 300 mA RCD or less to prevent fire still seems odd, 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?
To be fair likely the current was line to neutral causing the distortion, and once we got line to earth it tripped, so really it was not 6.9 watt but a lot higher, but in real terms the leakage did trip the RCBO before there was a fire, so yes we know a RCD does help prevent fire, but since talking about up to 300 mA RCD's which is 69 watt, not sure how effective they would be?
DIYnot Local
Staff member
If you need to find a tradesperson to get your job done, please try our local search below, or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Are you a trade or supplier? You can create your listing free at DIYnot Local
Sponsored Links
