Grouping Factor is giving me a headache.

[Engineer54]

Of all the derating factors to be applied to circuits, the ''grouping'' factor always seems to be the one that most electrciians have trouble understanding and coming to terms with.... Strange!!

 

[LeeH]

Why is it strange?

 

[Chr!s]

Why is it strange?

I think its more a case in reality there is often a mixture of cable sizes and thats where electricians hit the stumbling blocks.

 

[D Skelton]

I think its more a case in reality there is often a mixture of cable sizes and thats where electricians hit the stumbling blocks.

I think it has more to do with electricians seeing a table in the BGB that says 'X', 'Y' and 'Z', so they think that they must do 'X', 'Y' and 'Z', without reading the text either preceding or following the aforementioned table.

Sadly, some of these electricians go on to become lecturers, who then teach prospective electricians to do the same thing.

It's a constant cycle that few manage to break free from unfortunately. This industry is rife with examples of the blind leading the blind.

 

[ipf]

I think it has more to do with electricians seeing a table in the BGB that says 'X', 'Y' and 'Z', so they think that they must do 'X', 'Y' and 'Z', without reading the text either preceding or following the aforementioned table.

Sadly, some of these electricians go on to become lecturers, who then teach prospective electricians to do the same thing.

It's a constant cycle that few manage to break free from unfortunately. This industry is rife with examples of the blind leading the blind.

Too true. When will someone have the gumption to add a final appendix headed 'Common Sense'.....Then it's a case of...'If in doubt, refer to the final appendix'

 

[LeeH]

I think it has more to do with electricians seeing a table in the BGB that says 'X', 'Y' and 'Z', so they think that they must do 'X', 'Y' and 'Z', without reading the text either preceding or following the aforementioned table.

Sadly, some of these electricians go on to become lecturers, who then teach prospective electricians to do the same thing.

It's a constant cycle that few manage to break free from unfortunately. This industry is rife with examples of the blind leading the blind.

I don't think that's quite the case here, but I'm sure that's true. Our tutor seemed a fountain of knowledge.

The design course had a fundamental floor, and thats the 12 sessions it's spread over. It needs to be double that so the finer points like the grouping can be looked into in greater detail.

I'm still not 100% on exactly what I can and cannot do RE grouping but I will read through the parts that have been pointed out.

 

[D Skelton]

I don't think that's quite the case here, but I'm sure that's true. Our tutor seemed a fountain of knowledge.

The design course had a fundamental floor, and thats the 12 sessions it's spread over. It needs to be double that so the finer points like the grouping can be looked into in greater detail.

I'm still not 100% on exactly what I can and cannot do RE grouping but I will read through the parts that have been pointed out.

Yes I'm sure that your tutor would know his stuff if he was teching the 2396. I doubt they let any old mug teach that qual, considering any old mug won't pass it. Like you say, he's probably just limited on the amount of detail he can go into. There's a lot that has to be covered on that course and I agree, it's too short.

What you can do re. grouping, is to say; "well, I have 10 circuits in one piece of trunking, after diversity I have worked out that only two of them will be subject to reasonably steady state operation at roughly 60% load factor, the rest will be subject to erratic loading and are not interdepandant of each other, including three which I can disregard altogether as they will operate at less than 30% of load factor. What I'll do is apply a group rating factor for say three cables as this errs on the side of caution and allows for the erratic loading of the five remaining cables but also takes into account the fact that the first two cables are only operating at 60% load factor."

This method isn't an exact science, but without looking further into heat transfer analysis and the thermal effusivity of those cables, this is all you can be expected to do on a design course. Take account of the load on those circuits, take account of how much time they are going to be spending under load and take account of how that load will vary from circuit to circuit at different times.

Another thing you can do, is ensure that every cable selected operates at less than 30% of load factor. Yes you'll likely end up with ten times the amount of cable but you could forget altogether about grouping then.

Finally, one other option is to take the grouping factor literally and apply it across the board. Yes you'll end up with ridiculously sized cables, but who cares, they can't mark you down for it.

The choice is entirely yours buddy, you are the designer now!

 

[Chr!s]

I don't think that's quite the case here, but I'm sure that's true. Our tutor seemed a fountain of knowledge.

The design course had a fundamental floor, and thats the 12 sessions it's spread over. It needs to be double that so the finer points like the grouping can be looked into in greater detail.

I'm still not 100% on exactly what I can and cannot do RE grouping but I will read through the parts that have been pointed out.

if you post an example of your containment, I can go over it with you, use the reception/ office and classroms on the ground floor elevation.

 

[LeeH]

Thanks Chris, I'll wait to see what feedback I get with the amendments I've done. And be in touch if I'm struggling.

 

[Chr!s]

Thanks Chris, I'll wait to see what feedback I get with the amendments I've done. And be in touch if I'm struggling.

No worries

just remember, limit capacity to max 5 circuits, excluding circuits carrying les than 30%.

Its all to do with temperature, use the correct methods in appendix 4, then apply a final check for overall estimatex group temperature.

If you still have problems, pm me if you like and ill go over some of the techniques used.

Chris