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Spur Off Supply, Regs Number ?


MrBenn

Question

I understand that you can protect a cable from the field end in the example of:

a spur of a ring protected by a single socket 13ax2 plug tops or fcu 13a

I wanted to know if theres a written regs or guidance in another book regarding supply of control wiring from larger supplies. I.e.

Emergency stop contactor being fused down but supplied by 100a fed to the distribution board. I've explained the theory but my supervisor wanted written guidance which I'm struggling to find maybe Ive missed a keyword?  whilst on the subject sizing the cable feeding the fuse carrier/mcb is it a case of working out the max allowed per meter ? Ive noticed a lot of different sizes in panels and control systems. .   I hope this makes sense. Cheers in advanced

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15 answers to this question

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Where ever there is a reduction in the current carrying capacity of a conductor, there must be an OCPD fitted at that point. [basically that is, is more complicated than that] The cable is sized to suit the OCPD you chose and both, [cable and OCPD] have to be suited for the current the circuit is intended to carry.

 

Also, not sure what you mean, but as explained in the first sentence above, a cable must be protected at the SUPPLY end. There are NO exceptions to this.....

 

Cannot remember the reg number, but it is in there under "protection from overcurrent"

 

john...

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434.2.1 allows for the OCPD to be fitted other than at a change of CSA providing the length is no more than 3 metres and you take steps to reduce the risk of damage to the cable.

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Hi ProDave,

 

That is why i said "Basically" could not be arsed to type all that out!!

 

Interestingly enough though, the reg you give is the one that the DNO derive their "3 meter long tails" rule from i believe..

 

john..

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It is App' and there is a bit more to the intent of the reg, and it is in other standards too.

As far as control systems go, BS7671 may well not apply.

So, you could be looking in the wrong place.

Control panels for machinery are excluded from BS7671 as they are covered by other standards and are also covered by Statute Law requirements.

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would you want a fuse on a safety device that means it could fail to operate when needed? If you go down that line you would need to arrange circuits controls so that if small fuse blows it shuts down the machine

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Yes indeed, there is a LOT more to designing the controls for a machine than there is to designing an electrical installation. for a start the regs book is a LOT more expensive.

 

I used to do it years ago but haven't touched machine design for over 12 years now so I'm well out of the loop and wouldn't attempt it now.

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would you want a fuse on a safety device that means it could fail to operate when needed? If you go down that line you would need to arrange circuits controls so that if small fuse blows it shuts down the machine

That's exactly what you do. Controls fuse on safety circuit blows, machinery e-stops.

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Hi Paul, Yes "fail safe" is what is required. I think it boils down to FMEA analysis as to figure out the effects of a fault if one were to occur.

 

To go back the the "3 meter" rule, as it involves extra mechanical protection, at a guess i would think that the "extra intent" was aimed at TT systems, in that if you DID have an earth fault along the course of the 3 meters, you would be liable to have some VERY live exposed conductive parts....

 

john...

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Hi ProDave,

 

That is why i said "Basically" could not be arsed to type all that out!!

 

Interestingly enough though, the reg you give is the one that the DNO derive their "3 meter long tails" rule from i believe..

 

john..

I'm not sure that reg is the reason for 3 metre meter tails. After all the tails from the meter to the CU are rated for the full supply current and protected by the DNO's fuse regardless of how long they are. And there is no change of CSA involved either.

 

It's an additional isolator you are required to fit to long tails not additional OCPD so it's a different rule entirely. It just happens to be the same 3 metres.

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Err, no, you are not allowed to use the Cutout fuse to protect your installation, it is there purely to protect the service cable from overload. Anymore than three meters tail length, and you WILL need overcurrent protection..

 

To quote Eon;

 

"The customer’s consumer unit must be located so that the conductors from the electricity cut-out are no longer than 3m in order to comply with BS7671 regulations. If the unit exceeds this requirement, then suitable switch fused protection must be installed. This protective device must still be installed within the 3m limit"

 

john..

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I understand that you can protect a cable from the field end in the example of:

a spur of a ring protected by a single socket 13ax2 plug tops or fcu 13a

I wanted to know if theres a written regs or guidance in another book regarding supply of control wiring from larger supplies. I.e.

Emergency stop contactor being fused down but supplied by 100a fed to the distribution board. I've explained the theory but my supervisor wanted written guidance which I'm struggling to find maybe Ive missed a keyword?  whilst on the subject sizing the cable feeding the fuse carrier/mcb is it a case of working out the max allowed per meter ? Ive noticed a lot of different sizes in panels and control systems. .   I hope this makes sense. Cheers in advanced

Dragging us back to the OP  ......if I understand what Mr Benn is saying ,  as far as I know , to wire in your control / safety circuit  you can tap off a  100A phase in your contactor with , say , a short bit of  4mm to feed a control fuse or MCB which in turn , protects your 1.5mm global stop button wiring  .

 

This was always in the regs from the 13th as I remember,  much the same as smaller CSA cable within, say, a 500A  busbar chamber ...short 16mm tails would feed a  60A Sw/fuse . 

The likelyhood of them ever being damaged or short circuited inside there is minute.  

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Err, no, you are not allowed to use the Cutout fuse to protect your installation, it is there purely to protect the service cable from overload. Anymore than three meters tail length, and you WILL need overcurrent protection..

 

To quote Eon;

 

"The customer’s consumer unit must be located so that the conductors from the electricity cut-out are no longer than 3m in order to comply with BS7671 regulations. If the unit exceeds this requirement, then suitable switch fused protection must be installed. This protective device must still be installed within the 3m limit"

 

john..

I think that varies form DNO to DNO. It's not a BS7671 issue.  As far as 7671 is concerned, the 100A fuse protects the cable, 7176 does not recognise you not being "allowed" to use a particular OCPD.

 

As I understand it, some DNO's just require an isolator, others require an isolator and another OCPD.

Dragging us back to the OP  ......if I understand what Mr Benn is saying ,  as far as I know , to wire in your control / safety circuit  you can tap off a  100A phase in your contactor with , say , a short bit of  4mm to feed a control fuse or MCB which in turn , protects your 1.5mm global stop button wiring  .

 

This was always in the regs from the 13th as I remember,  much the same as smaller CSA cable within, say, a 500A  busbar chamber ...short 16mm tails would feed a  60A Sw/fuse . 

The likelyhood of them ever being damaged or short circuited inside there is minute.  

That's clearly the situation that reg was intended to cover, but the wording is a bit wolley to enable it to apply to other situations as well.

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A bit wooly eh !!  Was it not ever so  ?  :innocent

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Hi ProDave,

 

Yes that is right enough, as you say, the DNO's just make up their own rules, otherwise they would not be able to have a 25 metre long 35mm service cable connected directly to a 185mm LV main..!!

 

One rule for them and one for everyone else!!

 

john..

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Not quite john, but almost.

There are different rules for them, always have been,

Yes they do make up their own, because they have REAL Engineers to do this so that they comply with ESQCR.

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