What would you code this as.

[telectrix]

yes.youwillnotlikely see an overload but a short circuit is a different pot of herrings.

 

[Rockingit]

I'm 8 weeks into an apprenticeship and haven't covered it at college yet. Thought MCB's were for overload protection (whatever causes the overload) and no protection to people being shocked because it says it on several sites i've read, and that RCD's were solely to protect people and not equipment.

OK - that's fair enough. So, there's two things can happen to a cable - 1) too much current because of what's on the end of it and/or 2) too much current because of a short circuit between two or more conductors.

1) would be something like say a motor getting jammed (let's forget about overload limiters for now) and suddenly you go from a device that normally draws 50A's to suddenly drawing many times that, above and beyond the capabilities of the cable that's feeding it. Or something like a distribution board with too much plugged in. So that's an overload situation.

2) would be (normally caused by damage or poor installation) a dead short between say Live and Neutral (or any other combination). When that happens there is an instantaneous massive amount of current that is limited only by the type and size of the incoming supply, together with the actual resistance of the copper in the cable. In a domestic situation that might typically be 800-900A or in a commercial setting with three phase supply potentially many thousands of amps. That's fault protection.

A normal MCB / MCCB deals with both of these because the internal mechanism consists of two parts - one is a thermal trip (bi-metalic strip) that heats up slowly as an overload happens and the other is an electromagnetic device which shuts off much faster in the event of a fault situation.

In either situation, you don't want the cable catching fire because current = heat.

 

[Electrics]

OK - that's fair enough. So, there's two things can happen to a cable - 1) too much current because of what's on the end of it and/or 2) too much current because of a short circuit between two or more conductors.

1) would be something like say a motor getting jammed (let's forget about overload limiters for now) and suddenly you go from a device that normally draws 50A's to suddenly drawing many times that, above and beyond the capabilities of the cable that's feeding it. Or something like a distribution board with too much plugged in. So that's an overload situation.

2) would be (normally caused by damage or poor installation) a dead short between say Live and Neutral (or any other combination). When that happens there is an instantaneous massive amount of current that is limited only by the type and size of the incoming supply, together with the actual resistance of the copper in the cable. In a domestic situation that might typically be 800-900A or in a commercial setting with three phase supply potentially many thousands of amps. That's fault protection.

A normal MCB / MCCB deals with both of these because the internal mechanism consists of two parts - one is a thermal trip (bi-metalic strip) that heats up slowly as an overload happens and the other is an electromagnetic device which shuts off much faster in the event of a fault situation.

In either situation, you don't want the cable catching fire because current = heat.

??

Amazing thanks

 

[Raptor0014]

I know that i was literally just asking about the OP's assertion that the cable could pull more than it's rated for which in the absence of a fault i didn't think it was actually possible at all because of the MCB protecting whatever is drawing amperage through it.

My question was always from the point of view of there being no fault as i assumed from his wording that OP was concerned that the cable could pull 80a because the main fuse was 80a which to me seems like an irrelevance - in the absence of a fault it doesn't seem like that matters because 80a can only be pulled by whatever is in the shed which is MCB protected at 16a so it would never happen.

Not made myself very clear in what i mean but hey ho.

Apologies if I have confused you as I maybe wasn't clear in my comment.

I said the cable had the potential to pull 80a (or more if it was shorted in some way) when it is only rated for 47a. By saying it has the potential I was inferring in the event of a fault. Not in normal use. I know it won't pull more than 22a in normal use and that would be if the sockets were pulling the full 16a and the lights a full 6a. Which is extremely unlikely.

Something I was told by a friend who has been a spark for over 30yrs and I have carried this forward with me during my courses, exams, assessments and for any type of electrical work I do, be it installations or inspections etc. I look at what is in front of me and ask myself "what will happen in the event of a fault"

As long as the answer is RCD/MCB etc will de-energise the circuit/installation before any chance of death, injury or damage can occur beyond that which caused the fault, then it passes.

There is a whole lot more to it than that, and I am still learning as I gain experience.

But I am never afraid to ask, and I'm sure I used to drive my instructors nuts, as well as my assessor for CPS. But all of them have always said they would rather I asked a question, take the pi55 out of me if it's one I 'should already know the answer to' and then give me the answer.

I often question as to the reason why, but that's for my understanding rather than because I don't know something I should.

It just seems these days some people think the second you are qualified you should know BS7671 inside out and have the answer to every single situation you face.

I guarantee that ANY person on here there will be something they don't know electrically, despite all their experience. Does it mean they aren't competent if they ask a question? No it doesn't.

What seems a basic question to one (highly experienced) person is not necessarily so to someone with less experience. Some on here would do well to remember that they were in my position once themselves.

 

[Electrics]

Apologies if I have confused you as I maybe wasn't clear in my comment.

I said the cable had the potential to pull 80a (or more if it was shorted in some way) when it is only rated for 47a. By saying it has the potential I was inferring in the event of a fault. Not in normal use. I know it won't pull more than 22a in normal use and that would be if the sockets were pulling the full 16a and the lights a full 6a. Which is extremely unlikely.

Something I was told by a friend who has been a spark for over 30yrs and I have carried this forward with me during my courses, exams, assessments and for any type of electrical work I do, be it installations or inspections etc. I look at what is in front of me and ask myself "what will happen in the event of a fault"

As long as the answer is RCD/MCB etc will de-energise the circuit/installation before any chance of death, injury or damage can occur beyond that which caused the fault, then it passes.

There is a whole lot more to it than that, and I am still learning as I gain experience.

But I am never afraid to ask, and I'm sure I used to drive my instructors nuts, as well as my assessor for CPS. But all of them have always said they would rather I asked a question, take the pi55 out of me if it's one I 'should already know the answer to' and then give me the answer.

I often question as to the reason why, but that's for my understanding rather than because I don't know something I should.

It just seems these days some people think the second you are qualified you should know BS7671 inside out and have the answer to every single situation you face.

I guarantee that ANY person on here there will be something they don't know electrically, despite all their experience. Does it mean they aren't competent if they ask a question? No it doesn't.

What seems a basic question to one (highly experienced) person is not necessarily so to someone with less experience. Some on here would do well to remember that they were in my position once themselves.

Great points.