What would you code this as.

[DefyG]

If the SWA gets damaged, ie L-E or L-N fault, I should imagine the current will be a lot more than 80A, possibly nearer 800A (irrespective of whatever mcb’s/circuits are connected downstream), resulting in a fried cable and an expensive replacement for the sake of proper over current protection.

 

[Vortigern]

We are talking short circuit @Electric i.e. 1Kv. You know, fault of neglible impedance, PSCC-PEFC????

 

[Julie.]

Im not saying these faults don't happen i'm asking about OP claiming the client was 'lucky' because the cable 'could pull 80a' because there's an 80a main fuse. I'm just talking about the installation as it is now not under a situation where it may develop a fault.

Isn't it only the equipment the other end that can pull 80a, which would never happen since it's lighting and sockets protected by 6 and 16 amp MCB's?

There is always the possibility of one of the downstream devices failing to trip - contacts weld together or the mechanism itself fails, upstream protection will limit the extent of an overload in these circumstances; nevertheless I would tend to agree, overload protection is probably not necessary on the swa, however fault protection still is, and as it's the same device its omission raises a c2.

The semantics aside the customer was lucky - especially given the numerous glaring issues that exist in merely this one photo.

 

[littlespark]

So you can do away with the 6A mcb for the lights as well as it will only be a fixed load of maybe 10W if LED? ?

It’s basic principles here…. MCB to protect the cable from fault as well as overload.

 

[Electrics]

So you can do away with the 6A mcb for the lights as well as it will only be a fixed load of maybe 10W if LED? ?

It’s basic principles here…. MCB to protect the cable from fault as well as overload.

I thought MCB's only protected overload and RCD's protected earth faults?

 

[Julie.]

I thought MCB's only protected overload and RCD's protected earth faults?

No

Rcds only protect for earth faults (actually unbalance), mcb will protect against overcurrent whatever the reason (phase - phase fault, phase - neutral fault, phase - earth/cpc fault and overload)

 

[7029 dave]

I thought MCB's only protected overload and RCD's protected earth faults?

Short circuit as well

 

[Electrics]

Thanks for the infos.

 

[Rockingit]

Is it just me or has this thread just revealed a tragic lack of understanding between fault protection and overload protection? This is basic stuff ladies and gents.

 

[loz2754]

Is it just me or has this thread just revealed a tragic lack of understanding between fault protection and overload protection? This is basic stuff ladies and gents.

True. But let's not forget that the main protagonists are trainees.

 

[Rockingit]

I sometimes wish I'd gone into teaching!

 

[Julie.]

I sometimes wish I'd gone into teaching!

I did!

Technical collage - electrical engineering and electrical installation

Left it rather quickly - hated it.

 

[7029 dave]

I did!

Technical collage - electrical engineering and electrical installation

Left it rather quickly - hated it.

Steady money I would have thought, my ex-wife worked as a lecturer at a college in Luton she was on good money.

 

[littlespark]

My wife told me last night i might have made a good teacher....i had to disagree.

I hate high school kids... except my own. Hated them when i was one, hate them now.
I just don't have the patience when some little ---- is talking over me.

 

[Julie.]

Steady money I would have thought, my ex-wife worked as a lecturer at a college in Luton she was on good money.

It wasn't for me, I pretty much gave up after dealing with one student, he was useless at anything technical - but was in an onc technical stream.

One day I stepped in to a practical class as the lecturer was ill, I shown them how to do some simple conduit stuff, then checked up on their progress, this same "muppet" had understood perfectly, was pretty much as good as anyone well versed at it, and later on I noted his wiring, and understanding was spot on, I asked him why he was on the technical side when his craft stuff was great, and he clearly enjoyed it - just a "don't know shrug".

I asked the department head why he was put in the technical stream for which he was totally unsuitable, rather than craft which suited him completely and had spaces.

Answer "we need the numbers for the technical course"

I got a job elsewhere and moved on!

The college didn't want me to leave, as it was working out as far as students and they were concerned, but I had no appetite after that.

 

[Electrics]

Is it just me or has this thread just revealed a tragic lack of understanding between fault protection and overload protection? This is basic stuff ladies and gents.

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.

 

[telectrix]

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.

true, but a MCB downstream of the cable(SWA in your case) cannot protect the SWA if it were to short out for any reason. the MCB can only protect cable downstream of itself.

 

[Electrics]

true, but a MCB downstream of the cable(SWA in your case) cannot protect the SWA if it were to short out for any reason. the MCB can only protect cable downstream of itself.

I know that, my question was based on the OP claiming that the cable could potentially pull 80a. I was asking how can it when the only thing actually pulling any amps (in the absence of a fault) has an MCB that won't let it pull anywhere near enough to create a dangerous situation?

It's a case of crossed wires in this thread but too difficult to iron out over text so best to leave it there.

 

[telectrix]

I know that, my question was based on the OP claiming that the cable could potentially pull 80a. I was asking how can it when the only thing actually pulling any amps (in the absence of a fault) has an MCB that won't let it pull anywhere near enough to create a dangerous situation?

It's a case of crossed wires in this thread but too difficult to iron out over text so best to leave it there.

just imagine a kitchen fitter screwing into the SWA before the MCB. it should be OKas he'll wrap some tape round it. ??

 

[Electrics]

just imagine a kitchen fitter screwing into the SWA before the MCB. it should be OKas he'll wrap some tape round it. ??

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.

 

[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.