Theoretically... cables must be fused down when a lower size cable is connected to a larger sized ca

[assegayer]

Now I completely understand why cables must be fused down when a lower size cable is connected to a larger sized cable. To ensure the cable does not burn out from current that it cannot take.
Yes i know this

Now current doesnt go anywhere it isnt in demand for so if a spur off of say a 6mm for a aircondition unit someone was to take a 1.5mm off that for a light.In my thinking it would work fine as none of the current being pulled by the airconditon unit is going to go down the cable for the light.

Again i know you have to fuse down but im just speaking theoretically

 

[Darkwood]

@assegayer

In theory having such a fixed load would not cause issue where as the fact of the matter is the regulations do have clauses to allow this if certain conditions are met, on the short circuit issue, say the lamp had a N/L fault then this is where issues may arise, if you can ensure the ZS still meets the requirements at the light fitting then this should not cause issue either.

In certain scenarios as I just mentioned and not usually in the domestic scene, it is quite acceptable to have a OCPD rated higher than the CCC of the cable when connecting certain fixed loads, this can be essential for material and installations costs in larger installs like industrial where the inrush of a fixed load may warrant a larger OCPD as all that is required is short circuit protection due to the fixed nature of the load, other regulations still need to be obeyed like zs values and this is where a balancing act is often needed or you indeed have to increase the CSA if compliance cannot be met, or additional earthing, whichever is the most cost effective.

In your scenario, a reduction in cable size would require some sort of fusing down unless the OCPD is already sized to the smallest of the cables on the circuit.

 

[Richard Burns]

Thank you, but in the case of a short circuit wouldn't it only affect the 6mm cable in the situation described and not the 1.5mm stabbed of the 6mm. also don't MCBs detect short circuits whether big or small and trip anyway.

If the short circuit line to neutral occurred because of damage to the, say, far end of the 1.5mm², e.g. a metal disc pushed into the bayonet lampholder; at that point the energy of the short circuit will pass along the 1.5mm² cable back to the 6mm² cable for a brief period of time until the circuit breaker trips. In this case the time to taken to interrupt the fault has to be shorter than k²S²/I², in domestic this is very likely to be the case but worth checking, perhaps, if you are designing something like this.

 

[Darkwood]

@Richard Burns ...

I do question your overload scenario you mention with a high impedance fault, theorectically I agree it can happen but chances to meet these conditions are so small and the nature of most high impedance faults is such that they are current limiting shows why that it is often omitted as a concern hence we have regulations to allow higher OCPD's than that of the CCC of cables when used on fixed loads, if high impedance faults were considered an issue then this regulation would not exist.

 

[assegayer]

@darkwood
@Richardburns

Thank you both for your in depth explanations. So in conclusion if all Zs values are met it could probably be safe and sound

 

[Richard Burns]

I agree it is not likely but assegayer did ask what could cause an overload and this is probably the only way it could happen. Certainly there is no requirement for it to be accounted for in the regulations.

 

[Richard Burns]

@darkwood
@Richardburns

Thank you both for your in depth explanations. So in conclusion if all Zs values are met it could probably be safe and sound

Probably but not generally a good design in a domestic situation where there is really no reason not to provide overload protection.

 

[assegayer]

I agree it is not likely but assegayer did ask what could cause an overload and this is probably the only way it could happen. Certainly there is no requirement for it to be accounted for in the regulations.

This is exactly why i asked the question, because i always hear people saying in fault conditions but they cant even tell me what fault conditions other then short circuits. thanks

 

[assegayer]

The situation i was thinking was if someone wanted a light outside and the house inside is all concrete ceilings and walls etc which do not want to get wrecked. But you had a perfectly good ac unit outside with its dedicated circuit it would be much easier and less hassle to just stab of that. And as mentioned earlier a fcu would jsut not be practical.

 

[Richard Burns]

The situation i was thinking was if someone wanted a light outside and the house inside is all concrete ceilings and walls etc which do not want to get wrecked. But you had a perfectly good ac unit outside with its dedicated circuit it would be much easier and less hassle to just stab of that. And as mentioned earlier a fcu would just not be practical.

Weatherproof FCU exist and you could connect the 6mm² together in the back of the box and have a short say 4mm² spur off to the FCU terminals.

 

[Darkwood]

@darkwood
@Richardburns

Thank you both for your in depth explanations. So in conclusion if all Zs values are met it could probably be safe and sound

In theory yes, meeting regulations no, the set-up itself would mean you could only really achieve this only with a short 1.5 extention to the lamp, the higher resistance of the 1.5 in comparison to the 6mm against the lower permitted zs allowance often stops you doing this, although the cowboys out their who do do this kind of poor work are not bothered about zs if they even know what it meant.

EDIT... in conclusion that would bring us back to it's likely not to meet zs so the smaller cable would suffer damage in a fault and could negatively effect the properties and di-electric strength of the insulation and sometimes the sheath.

 

[Andy78]

Haven't fully read the thread so don't know if this has been mentioned, but the regs allow a change in cable size without fusing down at the point of size reduction if the circuit ocpd is rated to protect the smallest cable size in the circuit. This could be the easiest way to protect the 1.5mm cable in this situation.

 

[Darkwood]

@Andy78 ... Yes I brought that up in a recent post although it's an unlikely option when the 6mm is probably running a heavy load to start with but acknowledge we are talking theorectical here.

 

[Andy78]

@Andy78 ... Yes I brought that up in a recent post although it's an unlikely option when the 6mm is probably running a heavy load to start with but acknowledge we are talking theorectical here.

Ah sorry, that's me skim reading. I thought the situation was a repurposing of a former 6mm sitcom supply.

 

[ElectroChem]

So, if we were to hypothetically say that I had already done this dodgy spur, what sort of hypothetical justifications should I use to hypothetically convince myself I haven't left a firetrap behind?

 

[Richard Burns]

So, if we were to hypothetically say that I had already done this dodgy spur, what sort of hypothetical justifications should I use to hypothetically convince myself I haven't left a firetrap behind?

There are times when theory and practice meet; it is at this point that everything goes wrong! (hypothetically)