Best practices

0.48 ohms is about 475 amps fault current for a 230V LE voltage at the origin.

Looking at the bussman curves, that could take upto 8 seconds to blow or be faster than 2.

So it fails the 5 second limit in the regs. and is not safe for a well earthed person to touch while that fuse is blowing.  Mind you, 5 seconds is also fatal if you touch it while the faut is on but the fuse not yet blown.

In my mind, a small additional risk.

I;d also worry about the voltage drop - how much of this is R1, an how much R2 - you may be looking at 30V drop at full load. I presume that the motor takes rather less than 100A once up to speed.

Mike.

Under serious fault conditions and Voltage droop, or where there is additional resistance in the fault path under fault conditions,  will not the fault current be reduced and the disconnection time seriously lengthened? Where are the H.R.C. back up motor fuses?

Z.

where is additional resistance in the fault path under fault conditions,  will not the fault current be reduced and the disconnection time seriously lengthened?

Oh yes ..(Churchill insurance voice). But the voltage at point of fault, already set by the R1R2 voltage division, probably to about 100-150 v if live cores and CPC have similar cross-sections will then be further reduced as the voltage divider is now R1 + R_fault dividing against R2. If you can get the exposed voltage  below 50V the rules of the game suggest you can safely  lengthen the ADS operation time to ‘never’ without danger to life.

In reality I'd be very nervous about getting close to that lower limit, especially with things like  pump repairs, where wet hands could mean the body to metal contact may be lower resistance  than expected, and a lower threshold, e.g. 25V would be safer than 50.

  Equally, exposing a dangerous voltage for 5 seconds, or even ten is fine if no one is there when the fault comes on. If they are then the 5 secs of the regs  is not really adequate protection anyway, and earth fault relays RCDs etc are our weapons of choice.

Mike

MrJack96: 
 

The example I had was a motor circuit feed by 100amp bs88-2 fuses the Zdb was 0.20ohms and the R1+R2 was 0.28   ohms the cable from the panel to the local isolator uses the SWA as an earth. was in a situation where I refused to switch it on as it Doesn’t comply but Was seen as being a Jobs worth. 

Is there no other protection for the motor which would disconnect it in case of an earth fault which could make it live for long periods?

I can not imagine any scenario where a L. to E. fault inside a motor of a “bolted fault” type of negligible impedance, that would operate 100 Amp fuses, could occur, except perhaps the unlikely shorting of supply cables inside the motor terminal box.

More information needed please.

Z.

The situation is not quite as you see it MrJack96. I am not surprised you are being seen as a “jobsworth”, as you are effectively doing just that. In situations like this, you are not at the end of the chain of responsibility, and in fact, you can, and should, easily shift it elsewhere. This is done by reporting in writing to your supervisor or manager the details of the problem and ask them for clarification of what should be done about it. I suggest the best way to report is using the BS7671 inspection report form, you only need to fill in the parts relevant to the problem, sign and date it as the inspector, copy it, and send the original to the manager etc. Keep the copy, the original may be filed or lost and a question could be asked later!

It is worth noting that you need to be careful that your report is accurate, particularly measured values such as Zs and R1+R2, and you refer to the regulation and table numbers which are not complied with. In theory (and legally) your employer should have some kind of incident reporting scheme in place, and a mechanism to ensure that reports are dealt with properly.

As far as your actual worries go, the SWA armour should be fine as the CPC, its resistance is less than the conductors up to at least 50mm2 size if it is a BS type. The Zs max for 63A BS88s is 0.68 ohms for 5 seconds and 0.36 ohms for 0.4 seconds disconnection (Table 41-2 and 41-4), so you are about halfway between the two, about 3 seconds. The question to ask “is this a distribution circuit or a final circuit?”. This can be a bit tricky to answer, and really you need to know what the original EIC says. However, if there are any more CPDs in the circuit to the motor you probably fall under 5 seconds disconnection and you have not told us the size of the motor but I suspect it is less than 40kW, thus not consuming 63A! 

Thus your finding of “unsafe conditions” may well not be valid, the IR does not really matter and the disconnection of about 4 seconds (assuming your measurements are accurate, they may be quite a way out) may be quite valid if this is considered a distribution circuit. Did you disconnect the machine to get the IR? It is very likely that a piece of equipment can have a much lower value, and your BS7671 source is out of scope outside of the fixed wiring!

I have no idea of your level of experience, but you need to realise that safety is a matter for a chain of responsibility, which is fully explained in the relevant legislation. Your duty ends if you make a report to the next step of the chain for a decision, it is certainly not up to you to decide to stop a factory because you find a relatively minor problem. That is a decision well above your pay grade! Some others who also post here seem to have a problem with this as well, it is brought on by many tutors who have the same worry and do not understand the principles of H&S. We often use the car MOT as an example, the inspector will give you a list of any faults and if serious will suggest that you do not drive away, he will NOT try to hide the keys! You are in the same position, you advise the boss of problems, he will tell you to fix them or not, and if you are not happy get the ignore instruction in writing or see the next in the line above him. The last resort is given to you on the factories act notice that should be posted, it is the report number for the HSE, but you can always find the number on the net. Any action against you for the report will then be wrongful sanction or dismissal. That is the “non-jobsworth” route and much safer for you!

David CEng etc.

Thank you fro your response David. my concern though was if I did turn on that bit of equipment that has poor ZS readings even if I’ve got it in writing surely being electrical skilled we are still responsible in the court of law? the motor was protected by 100amp fuses which are the only CPD on the circui. 

MrJack96: 
… surely being electrical skilled we are still responsible in the court of law?

This is a matter of law rather than engineering. Please see my posting above about vicarious liability. If you were completely and utterly irresponsible in energising an unsafe circuit and somebody died - perhaps with exposed live conductors (C1) - you might just be criminally liable for gross negligence manslaughter, otherwise your employer is responsible for your mistakes.

Put it this way, if I came to harm due to your negligence, how much could I get from you (what are you worth?) or your employer (what is the company worth?).

If you are self-employed, this is where professional indemnity insurance comes in.

MrJack96: 
 

Thank you fro your response David. my concern though was if I did turn on that bit of equipment that has poor ZS readings even if I’ve got it in writing surely being electrical skilled we are still responsible in the court of law? the motor was protected by 100amp fuses which are the only CPD on the circui. 

What is the rating of the motor please? kW h.p? three phase, single phase?

Z.

davezawadi (David Stone): 
 

The situation is not quite as you see it MrJack96. I am not surprised you are being seen as a “jobsworth”, as you are effectively doing just that. In situations like this, you are not at the end of the chain of responsibility, and in fact, you can, and should, easily shift it elsewhere. This is done by reporting in writing to your supervisor or manager the details of the problem and ask them for clarification of what should be done about it. I suggest the best way to report is using the BS7671 inspection report form, you only need to fill in the parts relevant to the problem, sign and date it as the inspector, copy it, and send the original to the manager etc. Keep the copy, the original may be filed or lost and a question could be asked later!

It is worth noting that you need to be careful that your report is accurate, particularly measured values such as Zs and R1+R2, and you refer to the regulation and table numbers which are not complied with. In theory (and legally) your employer should have some kind of incident reporting scheme in place, and a mechanism to ensure that reports are dealt with properly.

As far as your actual worries go, the SWA armour should be fine as the CPC, its resistance is less than the conductors up to at least 50mm2 size if it is a BS type. The Zs max for 63A BS88s is 0.68 ohms for 5 seconds and 0.36 ohms for 0.4 seconds disconnection (Table 41-2 and 41-4), so you are about halfway between the two, about 3 seconds. The question to ask “is this a distribution circuit or a final circuit?”. This can be a bit tricky to answer, and really you need to know what the original EIC says. However, if there are any more CPDs in the circuit to the motor you probably fall under 5 seconds disconnection and you have not told us the size of the motor but I suspect it is less than 40kW, thus not consuming 63A! 

Thus your finding of “unsafe conditions” may well not be valid, the IR does not really matter and the disconnection of about 4 seconds (assuming your measurements are accurate, they may be quite a way out) may be quite valid if this is considered a distribution circuit. Did you disconnect the machine to get the IR? It is very likely that a piece of equipment can have a much lower value, and your BS7671 source is out of scope outside of the fixed wiring!

I have no idea of your level of experience, but you need to realise that safety is a matter for a chain of responsibility, which is fully explained in the relevant legislation. Your duty ends if you make a report to the next step of the chain for a decision, it is certainly not up to you to decide to stop a factory because you find a relatively minor problem. That is a decision well above your pay grade! Some others who also post here seem to have a problem with this as well, it is brought on by many tutors who have the same worry and do not understand the principles of H&S. We often use the car MOT as an example, the inspector will give you a list of any faults and if serious will suggest that you do not drive away, he will NOT try to hide the keys! You are in the same position, you advise the boss of problems, he will tell you to fix them or not, and if you are not happy get the ignore instruction in writing or see the next in the line above him. The last resort is given to you on the factories act notice that should be posted, it is the report number for the HSE, but you can always find the number on the net. Any action against you for the report will then be wrongful sanction or dismissal. That is the “non-jobsworth” route and much safer for you!

David CEng etc.

I worked for a company once that supplied a company vehicle. The vehicle was legally unroadworthy. One day I refused to drive it. Who would have been fined if I was stopped by the police, me or the company?

Z.