Hello SparkyNinja. Your videos are very very informative. As an engineer from another industry, I am trying to get myself into this trade. Passing recently 17th E. and PAT, now I learn towards 2391 and it is easier with your videos:). I like your technical approach to the problems as myself love to understand everything. Keep going !! And by the way, nice new video presentation.
Very interesting, again! A couple of considerations. Obviously if the earth fault loop impedance path is reduced by parallel paths a greater fault current will flow. But I don't think this has any affect on the the sizing of the CPC because the additional fault current will be proportionally shared with the cause of the parallel path. I think the important consideration when there are a lot of parallel paths, say in an industrial installation with metal containment fixed to structural steel, is being as sure as you can be that the CPC is in fact intact, and not just fortuitous due to the parallel paths. As you said, a parallel path may in some circumstances be removed, or reduced at a later date. The other point is that calculation only takes place during the design process as you said, this is done on a bit of pare, a spreadsheet or in Amtech or similar. But measuring Ze and R1+R2 and adding together is still a measurement, it's just taken in two measurements. Now we can all have a view on the accuracy of the measurement using a DC test method, and it obviously only measures resistance. But is direct measurement I'd Zs any more accurate with an EFLI tester? It might be as its measuring with AC at a higher current, but if you have an RCD in circuit and are using a no-trip test, especially two wire (Megger), it could well be a fruit machine test result, due to noise, etc. So there is room for debate, but I would argue that both method are in fact measurements - you are using a meter for both. Actual fact, at Elex I was looking at the new MFT from Megger that claims to be able to eradicate false readings when in no trip mode ( only for three wire method I think), so that would be a vast improvement. Thanks again for the video, very interesting.
Hi SN, thank you for you explanation ... it brought back dim memories of 'A' Level Electronic Systems and Physics together with Degree level Electronics and Electrical Generation from an Aviation perspective as well as Royal Air Force Engineering Specialist Training. I am sure that I have forgotten 'more' than I have ever learned! However, whilst an interesting 'canter' into details which many have long since committed to 'file 13', if I understand you correctly capacitive and inductive reactance both act to increase actual impedance; thus reducing the current flow in an AC circuit below the current predicted with the resistance 'measured' using DC. The approaches of 'measuring' or 'calculating' Zs and thence potential fault currents are surely both equally affected by this 'common' limitation? Whether 'measured' or 'calculated', both approaches are convenient, 'conservative' in their results and therefore 'adequate' for their purposes?
Thank you for creating this informed video with reference to specific regulations in BS 7671 and reference to GN3. My training has been that calculation of earth fault loop impedance by Ze + (R1 + R2) is acceptable and is preferable for the reason of not requiring access to live parts (and so reducing live working ref. EAWR 1989). The point you make about parallel paths is an interesting one, though surely if the impedance is reduced by parallel paths, the CPC will not be subjected to the full resulting fault current current, the fault current being proportional to the impedance of each part of the parallel path? The biggest question for me arising from this, is when carrying out dead tests for continuity of CPCs, you are supposed to test at every point in the circuit, verifying the results and recording the highest result, which you should expect to obtain at the extremity of the circuit (subject to various specifics). When obtaining your Zs by measurement, would you again be wanting to conduct the test at every point (every accessory - sockets, luminaires, switch positions etc) or is it considered acceptable to carry out measurement only at the part of the circuit where the highest R1 + R2 reading was obtained? Otherwise we will be spending a great deal of time taking things apart and putting them back together again for doing a second round of measurements.
If you are talking about a standard radial, I'd test r1+r2 and Zs at the end of line. Where you have a radial with multiple end of lines, test at each of them. For a ring, test everywhere.
Sir wonder if i can pick your brains? been asked to install wiring for 70 student flats all there will be is a ring main and a lighting circuit. i do not know what supply is going in but wondered what would be the easier way of installing this. was thinking submains to each room? whats your thinking many thanks
That sounds like a big job 😲. I would approach it in the way you describe, with sub-mains to a small board in each - a little like you get in these 'affordable' hotels. If you find the pricing an issue for that number of consumer units then I would still do sub-mains but try to divide them up to the most suitable central positions.
Brilliant one Sparky Ninja. You know your stuff, real interesting in-depth reason why we shouldn't just do the calcs, Thanks for you very informative in depth Vids.
Hi... all good stuff...Thanks. One question ...If you had a higher than expected Zs reading...higher than the regs allow 1.1 /1.08 Ohms for a 32 Amp Ring Final Circuit.....and then you introduced an RCBO for protection....would the Zs then be 1667 Ohms for the RCBO for a 32Amp Ring Final Circuit or would the Zs be as the regs state?
Hiya, The Zs would stay as to the actual measurement. However if a 30 mA RCBO was introduced then we can look at regulation 411.3.2.5. This mentions that the requirement for the disconnection times may not be needed if a device is used to lower the supply below 50 V a.c. in a QUICKER time than those given in chapter 41. So say we look at a TN system 32 A final circuit, this has a disconnection time of 0.4 seconds in Table 41.1. If we were to introduce an RCBO of 30mA to this final circuit then at 5 x IΔn it will trip in 0.04 seconds, removing the supply. This is fine, but only if we know the RCBO will work in the required time and rating to keep the voltage below 50 V, and for that we need the 1667Ω. This is why we may record the alternative Zs of 1667Ω on reports. If the original Zs is in excess however, it may also be worth verifying the Zn between line and neutral, as if this resistance is too high to achieve sufficient fault current for disconnection then the RCD will not assist with this.
What about the inaccuracies of loop impedance testers. I own multiple and MFT even after they have all come back from calibration with a pass, they will give different readings on the same test. Even more so with non-trip tests. Surely r1+Rr in a way is more accurate
Hi sparky Ninja whilst watching this insightful video and in particular your part regarding independence differing from resistance it reminded me of a query I had a while ago regarding whether or not include the impedance introduced via a RCD into a circuit whilst measuring the zs value or to take a reading either side and deduct from max zs measured? My feeling is whatever impedance is introduced into the installation must be taken into account although I have seen as much as 0.30ohms from mk 61008 RCD,s before.would be good to hear your thoughts.richard
When I fill out certificates I record the highest measured value in the Zs box, 90% of the time this is Ze + R1 + R2. On your impedance explanation, I've noticed that live testing with some types of equipment connected to the circuit can give me erratic or different readings, especially with 3 wire low current tests. What exactly have we proved by conducting a live test if the reading is susceptible to change by connected equipment? I carry out ELI where it is safe to do so. I wont stick my probes where i cant see or reach easily, i wont leave lots of live accessories hanging off the wall on a packed building site, and i wont cram myself into a stupid small space with live conductors.
Until the regulations are more clear on the matter then the calculation method will still be used for years to come. In fact using the calculation often gives a higher zs result than a measured value but having rcds in circuits and measuring zs with the non trip 15mA test current can be inaccurate.
A lot of jobs I've been on won't allow measured ZS values as they incorporate parallel paths. PFC at origin should be measured don't belive DNO will supply you with this figure(I might be wrong though).
Hi SparkyNinja I recently sat and passed my 2391 with one of the contributors to BS7671 and on the course he was teaching us that we should always do Zs as much as possible by calculation, this is due to the reasons given in the Statutory Document of the Electricity at Work Regs which states that live working working should be avoided as far as reasonably practicable, it is therefore practicable to avoid live working by removing things such as socket faces to take live Zs readings and pull down ceiling roses to expose live parts when this can easily be avoided by just using the Calculated method
Yeah that's wrong I am afraid. Otherwise it would be written in the guidance to that effect. Saying that some of the guidance is incorrect anyway, or insufficient. Where's the Zloop value in there to verify short circuit currents on final circuits to achieve disconnection for example? The requirements of the Electricity at Work Regulation 4(2) is to prevent danger, all systems shall be maintained so as to prevent, so far as is reasonably practicable, such danger. It then states in 4(3) that every work activity, including operation, use and maintenance of a system and work near a system, shall be carried out in such a manner as not to give rise, so far as is reasonably practicable, to danger. It doesn't say that live work should be avoided, it says that work should not give rise to danger. It's a completely different thing. If the worker is skilled, has suitable control procedures, suitable instrumentation, tools etc then effective maintenance must take place. If an electrical incident was to occur, such as static shock on machinery, arc flash or in-direct contact.. the question will always be asked if it could have been done differently, and more accurately. I work in the food industry and see this every week - I can even name the HSE officers forcing 6 factories to measure EFLI's (mustn't calculate) and to actually test their RCD's monthly with an instrument at the moment. Choosing to calculate a Zs test every time is both avoiding maintenance and going against the intention of BS 7671. Higher incident energy levels are left undiscovered and reactance isn't compensated for. Try not to listen too much to self-righteous people throwing their backgrounds in to prove their point. I challenge these types daily at the moment (#e5 on LinkedIn) Have a good think about it and come to your own conclusions moving forward, it's your competence - no one else's! Oh, congratulations on passing BTW 👍
SparkyNinja I completely agree with you mate I always measure my values as I’m not being funny if I can remove a board cover to expose live parts to take a Ze Reading then I’m most certainly competent to drop a switch or fitting to take a reading hahah, I take readings then just compare them to my calculated values
I don't follow your logic about parallel paths, at 12 mins. The max current the MCB will pass before tripping will not increase, but the proportion of the current (supplied via the line conductor) returning via the cpc will be reduced thanks to the parallel path. I believe the calc uses 5 x MCB rating (for a type B)? So Amps in the calc will not change. The line conductor will suffer an increase in current, but that is not the issue.
The point is regarding where we use the adiabatic to design, not accounting for parallel paths and then consideration that fault current would increase with installed parallel paths which might put the system under stresses that haven't been taken into account.
You peroidic a property...or... do an install in a property......Then a plumber comes along, as part of a bathroom / kitchen re-fit, and all those tests and measurements you did are now wrong because the pipework is now plastic instead of copper...making any certificate issued useless. Do you think ALL plumbers should be made aware of the fact that "improving" the pipework / water supply system, that they are going to affect the electrical system too? I ask because.....I recently had this when the builder and home owner decided that a new water main was needed...from Lead ( and rightly so for health and safety reasons ) to plastic...as this would aid the water pressure in the property etc... Without considering all the potential electrical issues this may cause. One final point...If we measure Zs / Ze....Do the multifunction machines we generally use actually measure it, "Z" or are they simplifying it to simple OHMS Law "R"? I think the latter.
Appreciate you taking time to shine light on this and highlight consoderations I wouldn't have jumped to without your explanation. Great video.
Very good insight to probably one of the biggest topics electricians disagree on. Thanks and keep it up
Welcome back. Great presentation thanks
Hello SparkyNinja. Your videos are very very informative. As an engineer from another industry, I am trying to get myself into this trade. Passing recently 17th E. and PAT, now I learn towards 2391 and it is easier with your videos:). I like your technical approach to the problems as myself love to understand everything. Keep going !! And by the way, nice new video presentation.
Very interesting, again!
A couple of considerations. Obviously if the earth fault loop impedance path is reduced by parallel paths a greater fault current will flow. But I don't think this has any affect on the the sizing of the CPC because the additional fault current will be proportionally shared with the cause of the parallel path. I think the important consideration when there are a lot of parallel paths, say in an industrial installation with metal containment fixed to structural steel, is being as sure as you can be that the CPC is in fact intact, and not just fortuitous due to the parallel paths. As you said, a parallel path may in some circumstances be removed, or reduced at a later date.
The other point is that calculation only takes place during the design process as you said, this is done on a bit of pare, a spreadsheet or in Amtech or similar. But measuring Ze and R1+R2 and adding together is still a measurement, it's just taken in two measurements. Now we can all have a view on the accuracy of the measurement using a DC test method, and it obviously only measures resistance. But is direct measurement I'd Zs any more accurate with an EFLI tester? It might be as its measuring with AC at a higher current, but if you have an RCD in circuit and are using a no-trip test, especially two wire (Megger), it could well be a fruit machine test result, due to noise, etc. So there is room for debate, but I would argue that both method are in fact measurements - you are using a meter for both.
Actual fact, at Elex I was looking at the new MFT from Megger that claims to be able to eradicate false readings when in no trip mode ( only for three wire method I think), so that would be a vast improvement.
Thanks again for the video, very interesting.
Great points made Sparky Ninja. And some great calculation references.
Thanks for this, when I was at college we were told to only measure Zs from sockets and to calculate the rest.
great information as always as dan mentions great calculation references
Hi SN, thank you for you explanation ... it brought back dim memories of 'A' Level Electronic Systems and Physics together with Degree level Electronics and Electrical Generation from an Aviation perspective as well as Royal Air Force Engineering Specialist Training. I am sure that I have forgotten 'more' than I have ever learned! However, whilst an interesting 'canter' into details which many have long since committed to 'file 13', if I understand you correctly capacitive and inductive reactance both act to increase actual impedance; thus reducing the current flow in an AC circuit below the current predicted with the resistance 'measured' using DC. The approaches of 'measuring' or 'calculating' Zs and thence potential fault currents are surely both equally affected by this 'common' limitation? Whether 'measured' or 'calculated', both approaches are convenient, 'conservative' in their results and therefore 'adequate' for their purposes?
Another fantastic vid!!
Thank you for creating this informed video with reference to specific regulations in BS 7671 and reference to GN3. My training has been that calculation of earth fault loop impedance by Ze + (R1 + R2) is acceptable and is preferable for the reason of not requiring access to live parts (and so reducing live working ref. EAWR 1989).
The point you make about parallel paths is an interesting one, though surely if the impedance is reduced by parallel paths, the CPC will not be subjected to the full resulting fault current current, the fault current being proportional to the impedance of each part of the parallel path?
The biggest question for me arising from this, is when carrying out dead tests for continuity of CPCs, you are supposed to test at every point in the circuit, verifying the results and recording the highest result, which you should expect to obtain at the extremity of the circuit (subject to various specifics).
When obtaining your Zs by measurement, would you again be wanting to conduct the test at every point (every accessory - sockets, luminaires, switch positions etc) or is it considered acceptable to carry out measurement only at the part of the circuit where the highest R1 + R2 reading was obtained?
Otherwise we will be spending a great deal of time taking things apart and putting them back together again for doing a second round of measurements.
If you are talking about a standard radial, I'd test r1+r2 and Zs at the end of line. Where you have a radial with multiple end of lines, test at each of them. For a ring, test everywhere.
Sir wonder if i can pick your brains? been asked to install wiring for 70 student flats all there will be is a ring main and a lighting circuit. i do not know what supply is going in but wondered what would be the easier way of installing this. was thinking submains to each room? whats your thinking many thanks
That sounds like a big job 😲. I would approach it in the way you describe, with sub-mains to a small board in each - a little like you get in these 'affordable' hotels.
If you find the pricing an issue for that number of consumer units then I would still do sub-mains but try to divide them up to the most suitable central positions.
Minimum cpc calculations do you not use PFC at the DB which would be higher than anything out in the field.
Just reading through guidance notes 3 (18th edition) and bs7671 (18th edition) and what they both say about regulation 643.7.3 differs slightly.
Brilliant one Sparky Ninja. You know your stuff, real interesting in-depth reason why we shouldn't just do the calcs, Thanks for you very informative in depth Vids.
Similar point how bad are people for ze-zs=r1+r2 other than parallel paths
Hi... all good stuff...Thanks.
One question ...If you had a higher than expected Zs reading...higher than the regs allow 1.1 /1.08 Ohms for a 32 Amp Ring Final Circuit.....and then you introduced an RCBO for protection....would the Zs then be 1667 Ohms for the RCBO for a 32Amp Ring Final Circuit or would the Zs be as the regs state?
Hiya,
The Zs would stay as to the actual measurement. However if a 30 mA RCBO was introduced then we can look at regulation 411.3.2.5. This mentions that the requirement for the disconnection times may not be needed if a device is used to lower the supply below 50 V a.c. in a QUICKER time than those given in chapter 41.
So say we look at a TN system 32 A final circuit, this has a disconnection time of 0.4 seconds in Table 41.1. If we were to introduce an RCBO of 30mA to this final circuit then at 5 x IΔn it will trip in 0.04 seconds, removing the supply.
This is fine, but only if we know the RCBO will work in the required time and rating to keep the voltage below 50 V, and for that we need the 1667Ω. This is why we may record the alternative Zs of 1667Ω on reports.
If the original Zs is in excess however, it may also be worth verifying the Zn between line and neutral, as if this resistance is too high to achieve sufficient fault current for disconnection then the RCD will not assist with this.
What about the inaccuracies of loop impedance testers. I own multiple and MFT even after they have all come back from calibration with a pass, they will give different readings on the same test. Even more so with non-trip tests. Surely r1+Rr in a way is more accurate
I see earth fault loop impedance as a short circuit with is a purely resistive path ie ZE + R1 + R2 .
Hi sparky Ninja whilst watching this insightful video and in particular your part regarding independence differing from resistance it reminded me of a query I had a while ago regarding whether or not include the impedance introduced via a RCD into a circuit whilst measuring the zs value or to take a reading either side and deduct from max zs measured? My feeling is whatever impedance is introduced into the installation must be taken into account although I have seen as much as 0.30ohms from mk 61008 RCD,s before.would be good to hear your thoughts.richard
When I fill out certificates I record the highest measured value in the Zs box, 90% of the time this is Ze + R1 + R2.
On your impedance explanation, I've noticed that live testing with some types of equipment connected to the circuit can give me erratic or different readings, especially with 3 wire low current tests. What exactly have we proved by conducting a live test if the reading is susceptible to change by connected equipment?
I carry out ELI where it is safe to do so. I wont stick my probes where i cant see or reach easily, i wont leave lots of live accessories hanging off the wall on a packed building site, and i wont cram myself into a stupid small space with live conductors.
Sorry, a few predictive text typos in that last post!
Until the regulations are more clear on the matter then the calculation method will still be used for years to come.
In fact using the calculation often gives a higher zs result than a measured value but having rcds in circuits and measuring zs with the non trip 15mA test current can be inaccurate.
You can not inquire about Ze values. As the maximums are already in regs.
Why doesnt bs7671 just say you can or you cant , why is nothing in that book straight forward.
A lot of jobs I've been on won't allow measured ZS values as they incorporate parallel paths.
PFC at origin should be measured don't belive DNO will supply you with this figure(I might be wrong though).
Hi SparkyNinja I recently sat and passed my 2391 with one of the contributors to BS7671 and on the course he was teaching us that we should always do Zs as much as possible by calculation, this is due to the reasons given in the Statutory Document of the Electricity at Work Regs which states that live working working should be avoided as far as reasonably practicable, it is therefore practicable to avoid live working by removing things such as socket faces to take live Zs readings and pull down ceiling roses to expose live parts when this can easily be avoided by just using the Calculated method
Yeah that's wrong I am afraid. Otherwise it would be written in the guidance to that effect. Saying that some of the guidance is incorrect anyway, or insufficient. Where's the Zloop value in there to verify short circuit currents on final circuits to achieve disconnection for example?
The requirements of the Electricity at Work Regulation 4(2) is to prevent danger, all systems shall be maintained so as to prevent, so far as is reasonably practicable, such danger.
It then states in 4(3) that every work activity, including operation, use and maintenance of a system and work near a system, shall be carried out in such a manner as not to give rise, so far as is reasonably practicable, to danger.
It doesn't say that live work should be avoided, it says that work should not give rise to danger. It's a completely different thing. If the worker is skilled, has suitable control procedures, suitable instrumentation, tools etc then effective maintenance must take place.
If an electrical incident was to occur, such as static shock on machinery, arc flash or in-direct contact.. the question will always be asked if it could have been done differently, and more accurately. I work in the food industry and see this every week - I can even name the HSE officers forcing 6 factories to measure EFLI's (mustn't calculate) and to actually test their RCD's monthly with an instrument at the moment.
Choosing to calculate a Zs test every time is both avoiding maintenance and going against the intention of BS 7671. Higher incident energy levels are left undiscovered and reactance isn't compensated for.
Try not to listen too much to self-righteous people throwing their backgrounds in to prove their point. I challenge these types daily at the moment (#e5 on LinkedIn)
Have a good think about it and come to your own conclusions moving forward, it's your competence - no one else's!
Oh, congratulations on passing BTW 👍
SparkyNinja I completely agree with you mate I always measure my values as I’m not being funny if I can remove a board cover to expose live parts to take a Ze Reading then I’m most certainly competent to drop a switch or fitting to take a reading hahah, I take readings then just compare them to my calculated values
I don't follow your logic about parallel paths, at 12 mins. The max current the MCB will pass before tripping will not increase, but the proportion of the current (supplied via the line conductor) returning via the cpc will be reduced thanks to the parallel path. I believe the calc uses 5 x MCB rating (for a type B)?
So Amps in the calc will not change. The line conductor will suffer an increase in current, but that is not the issue.
The point is regarding where we use the adiabatic to design, not accounting for parallel paths and then consideration that fault current would increase with installed parallel paths which might put the system under stresses that haven't been taken into account.
You peroidic a property...or... do an install in a property......Then a plumber comes along, as part of a bathroom / kitchen re-fit, and all those tests and measurements you did are now wrong because the pipework is now plastic instead of copper...making any certificate issued useless.
Do you think ALL plumbers should be made aware of the fact that "improving" the pipework / water supply system, that they are going to affect the electrical system too?
I ask because.....I recently had this when the builder and home owner decided that a new water main was needed...from Lead ( and rightly so for health and safety reasons ) to plastic...as this would aid the water pressure in the property etc... Without considering all the potential electrical issues this may cause.
One final point...If we measure Zs / Ze....Do the multifunction machines we generally use actually measure it, "Z" or are they simplifying it to simple OHMS Law "R"? I think the latter.
Can you calculate R1 + R2?
From your desk when doing cable calculations at the design stage, yes. From measurements of an existing installation.. no. 👌
Yes