Even if you had, you would have concluded that 13A through 1.5mm² cable would give you a bit over 30 metres for a 5% drop, which would be more than enough in the vast majority of residential properties.
Kind Regards, John.
3A MCB/RCBOs
- Thread starter mikhailfaradayski
- Start date
Kind Regards, John.
Don't forget any applicable de-rating factors
Gents,
I think the point that is being missed here is that we tend to use the MCB as a combined overload/short circuit protective device. We often combine these functions into one device but we don't have to.
Sometimes overload protection could be omitted. Consider the immersion heater - its rated at, say, 3kW ie 13.04A @ 230V. Now, this device is either going to be functional or faulty. If its functional, it isn't going to suddenly start to pull, say, 19A - it can't. It cannot overload its circuit so we dont need to provide overload protection. We do, however, still need to provide short circuit protection. If its faulty then it will most likely fail short circuit or fail to earth.
If it fails short circuit then the current which flows will be limited by the impedance of the circuit & will approach the Psc - this will be several hundred amps. If it fails to earth, then if the installation is TNCS or TNS then the earth fault current will approach the short circuit current. If TT then the RCD should operate & the MCB would prob never operate anyway.
This is the reasoning behind saying that the protective device is to protect the cable - since by the time it is called into service the device has failed anyway - we just need to protect the cable from damage caused by the fault current.
Obviously, overload protection cant be omitted from any circuit which is liable to overload - ie. the load is not fixed at design time, such as circuits containing socket outlets.
If one studies the characteristic of an MCB one can see where these two functions have been combined, the instantaneous portion (the magnetic trip ie. shot circuit protection) is defined by the letter (eg. B, C, D etc), the overload portion is defined by the number, eg 16. The 16 bit operates on an IDMT curve such that the greater the load current is obove the rating, the faster that the device will operate until the instantaneous part takes over.
I think the point that is being missed here is that we tend to use the MCB as a combined overload/short circuit protective device. We often combine these functions into one device but we don't have to.
Sometimes overload protection could be omitted. Consider the immersion heater - its rated at, say, 3kW ie 13.04A @ 230V. Now, this device is either going to be functional or faulty. If its functional, it isn't going to suddenly start to pull, say, 19A - it can't. It cannot overload its circuit so we dont need to provide overload protection. We do, however, still need to provide short circuit protection. If its faulty then it will most likely fail short circuit or fail to earth.
If it fails short circuit then the current which flows will be limited by the impedance of the circuit & will approach the Psc - this will be several hundred amps. If it fails to earth, then if the installation is TNCS or TNS then the earth fault current will approach the short circuit current. If TT then the RCD should operate & the MCB would prob never operate anyway.
This is the reasoning behind saying that the protective device is to protect the cable - since by the time it is called into service the device has failed anyway - we just need to protect the cable from damage caused by the fault current.
Obviously, overload protection cant be omitted from any circuit which is liable to overload - ie. the load is not fixed at design time, such as circuits containing socket outlets.
If one studies the characteristic of an MCB one can see where these two functions have been combined, the instantaneous portion (the magnetic trip ie. shot circuit protection) is defined by the letter (eg. B, C, D etc), the overload portion is defined by the number, eg 16. The 16 bit operates on an IDMT curve such that the greater the load current is obove the rating, the faster that the device will operate until the instantaneous part takes over.
We've been over this ground at length in the past, and the argument you present would be fine in a world in which there was always as clear a distinction between fault and overload as you suggest - in particular, your argument assumes that the only sort of 'fault' is one of negligible impedance - i.e. a 'short' or 'bolted fault'.
The real world is not always as tidy as that. Faults (everyday meaning) that result in currents in what you would call the 'overload' range (i.e. faults of non-negigible impedance) can, and do, occur - extremely unlikely with an immersion element, I agree, but much more likely if, for example, motors or electronics are involved.
I can but repeat my philospohy that, unless someone can explain a downside to me (or unless we ever see the day when every electrical load has a built-in appropriate fuse), I'll continue to protect at the minimum level that is adequate for the fixed load in question, even if the cable concerned could be adequately protected with an appreciably higher rating of OPD. It that so silly?
Kind Regards, John.
You do have to be careful about the failure modes of the load in question. With some type of loads you might just not know what the failure modes are possible and you are forced to recommend that unless the manufacturer can be consulted and can tell you that the device is not able to cause an overload. that the MCB is changed to the correct value.
Motors are worthy of a separate mention, they are at risk of overload currents from stalling, etc but protection against this is normally down to a thermal overload in the motor starter, rather then the circuit protection back at the DB.
Exactly. In reality, who is going to consult the manufacturer for a discussion about possible failure modes for each and every load they wire into a circuit? In the absence of that, unless the answer is fairly obvious, the safest/simplest course is surely to assume that any load may, under certain circumstances, develop a fault which results in a moderate current, in what you would probably regard as the 'overload' range.
Indeed, but I would imagine that a high proportion of motors in domestic and DIY appliances/equipment probably don't have any built-in protection of their own - so, again, if in doubt, I would suggest one should assume that they could possibly result in 'overload' currents.
Kind Regards, John
Lets assume for a moment that this immersion heater isn't RCD protected. Further let us assume that the tank is earthed.
Now consider the immersion develops an insulation failure allowing current to flow into the tank via the water. This will increase the current draw by the immersion heater as the water is now in paralell with some part of the element. How much it will increase it by depends on the conductivity of the water in question (which varies a lot) and the location of the fault.
While in this state the immersion heater will still be functional. The water will continue to be heated and the thermostat will continue turning the heater on and off.
With many devices it is perfectly possible for them to be both functional and faulty.
Just because a device has failed doesn't mean we don't want to limit the damage done by that failure to both the device and it's surroundings. If a device is drawing far more current than it should be then that energy must be going somewhere, likely into making something very hot.
Yes an important function of the protective device is to protect the cable but depending on what if any itnernal protection the device has it may serve to limit the damage caused by internal faults in the device as well. As such if a manufacturer says their device should be protected by a protective device of no more than a given rating you should respect that.
Whilst it is difficult to argue against the principle of fusing down to the minimum for appliances, what does this involve?
It may be straight forward for single load circuits with FCUs all over the place and a supply of every rating of fuse. Do we stick to integers?
However, have you considered what this would mean on ring or radial final circuits supplying the rest of the house.
Do we fit plugs with 9A fuses for 2kW kettles, 7A for small vacuums, will 1A do for a 40W lamp?
The fact is that the fuse/mcb is to protect the circuit (cable and accessories) and the portable appliance on the end will be supplied with the necessarily rated flex and fixed equipment may stipulate fusing down because they are too cheaply made.
It may be straight forward for single load circuits with FCUs all over the place and a supply of every rating of fuse. Do we stick to integers?
However, have you considered what this would mean on ring or radial final circuits supplying the rest of the house.
Do we fit plugs with 9A fuses for 2kW kettles, 7A for small vacuums, will 1A do for a 40W lamp?
The fact is that the fuse/mcb is to protect the circuit (cable and accessories) and the portable appliance on the end will be supplied with the necessarily rated flex and fixed equipment may stipulate fusing down because they are too cheaply made.
How does it do that without the filament coming into contact with the outer earthed covering of the element, consequently blowing the fuse/mcb or burning itself out and ceasing to work?
I'm still to be convinced....
Going back to the immersion heater ....with its normal load current of 13.04A @ 230V, giving a working resistance of 17.64Ohms. Now, the mains can go upto 253V and still be within band, so our heater would pull 14.34A @253, so the smallest MCB we could consider would be 15A.
Now lets look at an earth fault. If the fault where to occur right at the 'live' terminal of the heater and you have an excellent TT earth (say, 20Ohms at the sub & 20Ohms for your rod), then the earth fault current would be 5.75A. Our total cicruit current is 13.04 + 5.75 = 18.79A. Or, in other words 25% over the rating of the MCB. An MCB to BS EN 60898 will not trip for such an overload. A current of 75A would be needed for the device to operate in under 5s. But an earth fault right on the terminals is very unlikey....if more likely to be somewhere along the length of the filament... making the fault current even less.
In the example above, close fusing wont have any better result than chosing a 20A device to protect the cable.
Perhaps the reason that a manufacturer stiplates that a device should be protected by a 3A fuse is because he knows that its internal wiring is in 0.75mm or even 0.5mm so wont be protected by a 6A MCB - again, protecting the wiring not the device. I repeat my orginal point - by the time that a fuse operates, the device is already faulty ... all we can do is limit damage to the infastructure.
Dont forget that any kind of fuse or MCB can't limit the fault current, only the length of time that it flows for. Once the fault has occured, the damage has already been done.
Going back to the immersion heater ....with its normal load current of 13.04A @ 230V, giving a working resistance of 17.64Ohms. Now, the mains can go upto 253V and still be within band, so our heater would pull 14.34A @253, so the smallest MCB we could consider would be 15A.
Now lets look at an earth fault. If the fault where to occur right at the 'live' terminal of the heater and you have an excellent TT earth (say, 20Ohms at the sub & 20Ohms for your rod), then the earth fault current would be 5.75A. Our total cicruit current is 13.04 + 5.75 = 18.79A. Or, in other words 25% over the rating of the MCB. An MCB to BS EN 60898 will not trip for such an overload. A current of 75A would be needed for the device to operate in under 5s. But an earth fault right on the terminals is very unlikey....if more likely to be somewhere along the length of the filament... making the fault current even less.
In the example above, close fusing wont have any better result than chosing a 20A device to protect the cable.
Perhaps the reason that a manufacturer stiplates that a device should be protected by a 3A fuse is because he knows that its internal wiring is in 0.75mm or even 0.5mm so wont be protected by a 6A MCB - again, protecting the wiring not the device. I repeat my orginal point - by the time that a fuse operates, the device is already faulty ... all we can do is limit damage to the infastructure.
Dont forget that any kind of fuse or MCB can't limit the fault current, only the length of time that it flows for. Once the fault has occured, the damage has already been done.
Indeed - if you open up most standard small extractor fans at look at what is on 'the other side' of the power connector block, it's not hard to see why they ask for a 3A fuse. In fact, I'm far from convinced that the wires, let alone the PCB tracks, are even 0.5mm², so even 3A might be too high a rating for the fuse.
I think we're getting close to semantics here. It is protecting current-carrying conductors within the device. In my language, that is really 'protecting the device', not just 'the cables'.
True ... but also don't forget (a) that, without close fusing, an 'abnormally high current' (for the device) may flow indefinitely, and (b) that, although some damage will already have been done when the primary 'fault' (abnormally high current for the device) first arises, further 'collateral damage' may develop if that abnormal state is allowed to persist. For example, a current of 8.7A (I2 of a B6 MCB) through the internal conductors and PCB tracks of many a 'device' (like the fan) could cause smoke, and even flames, long before the 1 hour potentially required for the OPD to operate had elapsed.
Kind Regards, John.
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