College Theory Question

[RyanM]

Could anyone help with this question please (not written like-for-like):

A lighting circuit in an office complex is wired in 1.5mm T+E, in a ring main. The total load is 9.8A, 240v, 2352W, with a 10A MCB protecting it.

From a recent thermo survey, it show's the MCB getting too hot. What corrective measures could be done?

I'm told there are a few correct answers, and I *think* the following could be done:
- Split the load in two, and have two separate ring mains. Although this may be too far fetched?
- Replace the MCB incase it is defective? I'm not sure if MCB's are designed to run very close to the rating??
- Up-rate the MCB to 16A, as this would still give the 1.5mm cable the protection it needs?


Do they sound feasible for me to submit, or can you think of anything else?


Thanks a lot

 

[riveralt]

You cannot have an A1 circuit with 1.5mm T&E. The only occasion you would have a RFC with 1.5mm cable would be with mineral insulated and the circuit would be protected by a 30/32Amp MCB.

Are you sure the T&E cable size is not 2.5mm?

The fact that you are using 240v instead of 230v suggests you are not following the 17th edition of BS7671. Using 230v gives you just over 10Amps.

Reason the MCB is getting hot could be a loose conductor in the terminal.

Increasing the size of the MCB will depend on the size of the cable, the length of the cable run, how the cable is run etc.

Read BS7671.

 

[securespark]

It's lighting, not power....

 

[Spark123]

It is a bit of an odd question as a 1.5mm T&E Ring Final Circuit isn't recognised by the current edition of the IEE regs.

The easiest way would be to convert to at least more than one 6A or 10A radial lighting circuit, of course the installation method, volt drop and max EFLI would have to be taken into account.

 

[EFLImpudence]

- Split the load in two, and have two separate ring mains. Although this may be too far fetched?

Could be split into two radials.

- Replace the MCB incase it is defective?

Could do. More likely loose connection, as already said.

I'm not sure if MCB's are designed to run very close to the rating?

They are.

- Up-rate the MCB to 16A, as this would still give the 1.5mm cable the protection it needs?

You could do, especially if it is a Ring Final.


If it says in the question 'ring main' then this is an incorrect term.

Why did you not quote the question word for word?

I know it asks "What corrective measures could be done?" but the correct procedure would be to discover the reason for the overheating, not just try something it could be.
Perhaps this is the answer they are seeking from you. I.e. No corrective measures before cause is found.

Applying one of your answers may disguise the true fault.

As has been said it is an odd question.

 

[PrenticeBoyofDerry]

I think the first corrective measure would be to tighten the terminals on the MCB.
If this then proves to have resolved your problem, I would then check that no damage has been done to the insulation of the conductors due to the over heating at the MCB.

 

[Paul_C]

You cannot have an A1 circuit with 1.5mm T&E. The only occasion you would have a RFC with 1.5mm cable would be with mineral insulated and the circuit would be protected by a 30/32Amp MCB.

A lighting ring final in 1.5 sq. mm cable isn't one of the standard circuit arrangements set out in the regs., but there's nothing to say that it can't be done. It's just a little unconventional.

 

[JohnW2]

A lighting ring final in 1.5 sq. mm cable isn't one of the standard circuit arrangements set out in the regs., but there's nothing to say that it can't be done. It's just a little unconventional.

That's very true, but I'm not sure that much would be gained, particularly since such a circuit would presumably not enjoy any of the 'concessions' in the regs relating to RFCs. For design purposes, one would presumably have to consider it as a case of 'conductors in parallel with unequal (and variable) current sharing'. I would have thought that, for all sorts of reasons, splitting into two or more radial lighting circuits would probably be preferable in the case which has been described.

Kind Regards, John.

 

[Paul_C]

Agreed on all counts John. It's not something I would do, but then I'm not a fan of the supposedly great British ring final circuit in the first place.

 

[JohnW2]

Agreed on all counts John. It's not something I would do, but then I'm not a fan of the supposedly great British ring final circuit in the first place.

Same here. I suppose that, if one is prepared to sit down and design from scratch, a lighting ring would be more ameniable to proper design than a standard socket RFC - since, unlike the latter, one would be working with defined loads at defined points in the ring. So many unknowns and uncertainties are associated with a socket RFC that a proper design process is not really possible - which I guess we tend to rely on what is 'allowed' by the regs.

Kind Regards, John.

 

[ban-all-sheds]

The problems with the traditional ring final circuit for BS1363 accessories stem from the fact that it allows a cable with Iz < In to be used.

Where (as in this case) there is a ring with Iz > In those problems go away.

A lighting ring final in 1.5 sq. mm cable isn't one of the standard circuit arrangements set out in the regs., but there's nothing to say that it can't be done. It's just a little unconventional.

On the contrary, I believe that it's quite common to use ring circuits for lighting circuits in commercial premises in order to reduce voltage drop.

 

[JohnW2]

A lighting ring final in 1.5 sq. mm cable isn't one of the standard circuit arrangements set out in the regs., but there's nothing to say that it can't be done. It's just a little unconventional.

On the contrary, I believe that it's quite common to use ring circuits for lighting circuits in commercial premises in order to reduce voltage drop.

I don't think what you and Paul said are really incompatible.

However, I still wonder whether a lighting ring is often going to be the best solution. Voltage drop would become much less of an issue if one split the circuit into two or more radials (each carrying less current), and I suppose one could contemplate going up to 2.5mm² (subject to termination being OK) if it were still an issue even then. 10A of lighting is quite a lot for one lighting circuit (about 40 55W fittings) and, apart from anything else, convenience (hence possibly 314 of regs) could be improved, and possibly cost of wiring reduced, by splitting the circuit into more bite-sized pieces.

As you've implied, one of the 'advantages' (aka 'problem!) of a standard reg-compliant RFC is that it allows a situation in which In>Iz, but (as you say) that's not applicable to what we're talking about here.

However, it's obviously all down to the designer to decide what is best in a given situation.

Kind Regards, John.

 

[riveralt]

In learning mode here... I know the OP's question related to an 'office complex', but in the terms of lighting circuits for a warehouse scenario, measuring 200m * 100m * 20m (height) with no natural light would you design say....

A number of radial final circuits each controlling one bank (row) of lamps - with each lamp plugged into its own socket protected by its own fuse.

A number of ring final circuits each controlling their own banks of lamps - with each lamp plugged into its own socket protected by its own fuse.

or a number of standard lighting radial circuits (no sockets) - with an appropriate cable size and MCB back to the main board.

Would the power factor influence such a choice - I'm sure there must be something in Part L or other requirement to get close to 1.

you can tell its quiet out there

 

[JohnW2]

In learning mode here... I know the OP's question related to an 'office complex', but in the terms of lighting circuits for a warehouse scenario, measuring 200m * 100m * 20m (height) with no natural light would you design say....
A number of radial final circuits each controlling one bank (row) of lamps - with each lamp plugged into its own socket protected by its own fuse.
A number of ring final circuits each controlling their own banks of lamps - with each lamp plugged into its own socket protected by its own fuse.
or a number of standard lighting radial circuits (no sockets) - with an appropriate cable size and MCB back to the main board.

I'm obviously also in learning mode since (although this is a scenario I never have considered, and never will have to consider), I don't think I would have considered the first or second of those options.

Would the power factor influence such a choice - I'm sure there must be something in Part L or other requirement to get close to 1.

I'm definitely in learning mode now. Why would the choice between those three options have any bearing on power factor?

Kind Regards, John.

 

[ban-all-sheds]

I don't think what you and Paul said are really incompatible.

A lighting circuit in an office complex

I believe that it's quite common to use ring circuits for lighting circuits in commercial premises in order to reduce voltage drop.

It's just a little unconventional.