Some Inspection and Testing questions for you

[bambubar]

Hello all,

I have recently begun carrying out more I&T jobs and have a few questions that I wanted to put to the forum.

1. Why are Zs readings, taken at the main switch disconnectors at distribution boards fed via UPS systems so high and how do these readings satisfy maximum permitted Zs values from their respective MCB?
2. When carrying out the safe isolation process, can a low resistance ohm meter be officially used in place of a voltage indicator/test lamp in order to prove that supplies are live or dead.
3. How is it determined what sizes the main protective bonding conductors should be that are supplying the respective gas and water supplies at additional buildings on a large multi building site? Are they conducive to the sizes of the main cables that supply the said additional buildings? (the same principal as the main gas and water supplies in the primary building).
4. If it is found that a lighting circuit has a cpc installed but has gone missing at a particular light, should this be noted as a C2 or C3 observation? (assuming the light is just a plastic pendant).
5. Is it permissible to have 3x2.5mm csa cables connected into the MCB of a 32A ring circuit if one has been tested and confirmed to have been used as radial tap off? 3 cables is obviously permissible at a socket or spur point so is it permissible if it is terminated at the MCB or would it be deemed as an overrated circuit?
6. If a radial socket or spur is bolted onto a distribution board (meaning the cable run of the circuit is contained within the board and also of a very short length) does it matter if the cable size is too small for the MCB that is protecting it or would it be classed as overrated? (the same as any other circuit).

Questions 5&6 in particular I've heard very mixed opinions about from other engineers so any clear feedback on these issues would be welcome.

Cheers for reading all.

 

[Richard Burns]

If the circuit cannot be safely disconnected when powered by the UPS then supplementary bonding to reduce the touch voltage to <50V would be required.
A low resistance ohmmeter will not function such as to indicate live or dead (many would fail or blow a fuse if connected to a live supply). It would also not meet the requirements of GS38 that specifies the construction of contact voltage testers. If you were meaning a voltmeter then the chance of inaccurate settings giving a false reading is considered dangerous and so they should not be used for proving safe isolation.
Main protective bonding conductors are sized in accordance with the size required for the installation earthing conductor for non PME supplies and in accordance with the supply neutral conductor at origin for PME supplies.
If the light is a class II light then there is no danger presented by the lack of an earth at the fitting but a danger may be presented if the supplying cable is not earthed. An earth should be run to every point of use so it would generally be recommend to improve the situation.
You may also have further lights along the circuit unearthed if there is a break. (but note that pendant cables from a pendant set do not require an earth).
Yes you can spur off a ring at the origin of the circuit. Care must be taken to ensure secure connection of the conductors (as in all cases).
If the socket or spur cannot take a current in excess of the rating of the cable then this is permissible, but generally not ideal in this case, as the cable cannot be overloaded by virtue of the load characteristics (i.e. the load cannot exceed 13A either because of the plug fuse or the FCU fuse). Minimum cable size of 1.5mm² for power circuits also applies. Generally it is simplest and best for basic situations like this to run a normally sized cable, but it is allowable to use a smaller cable than would be normally associated with the size of MCB.

 

[bambubar]

Thanks for the reply Richard. Just to clarify/expand on a couple of your points:

Q3. If a supply, from an existing installation was run to an additional building on a site in, for example a 35mm XLPE cable would this mean that the main water/gas supply to this additional building would need to be wired in a minimum of 10mm? If it was supplied using a 70mm csa cable it would need to be a minimum of 25mm? etc etc. Basically, the size of the bonding to the water/gas supply in the new building would depend on the sizing of the incoming supply cable to that building?

Q5. I thought that this was the case. However could you tell me where this is stated in BS7671 or in a I&T document so I can confidently re-iterate this point as it has caused many a debate in the past.

Q6. So does this mean that it is irrelevant if the final point of the circuit is contained within the distribution board (with the socket/spur bolted onto the side) or away from the board entirely and it also does not matter if the cable is too small for the MCB as long as the cable cannot have its load exceeded?
Another one of the reasons I asked this particular question about cables that run a short distance within an enclosure is because when I look inside busbar chambers there are a very diverse range of cable sizes supplying local isolators which then supply equipment within the installation. Does it not matter if the cable sizes to these isolators is very small when the fuse size within the main switch to the busbar chamber is very large because they are contained within an enclosure?

 

[Richard Burns]

Thanks for the reply Richard. Just to clarify/expand on a couple of your points:

Q3. If a supply, from an existing installation was run to an additional building on a site in, for example a 35mm XLPE cable would this mean that the main water/gas supply to this additional building would need to be wired in a minimum of 10mm? If it was supplied using a 70mm csa cable it would need to be a minimum of 25mm? etc etc. Basically, the size of the bonding to the water/gas supply in the new building would depend on the sizing of the incoming supply cable to that building?
No, It is as I stated in the answer; it is dependent on the size of the incoming installation supply at origin. So if the incoming supply neutral is 70mm² then any main bonding for the whole installation would need to be 25mm² minimum (on PME). Irrespective of the size of the sub main to a remote building.

Q5. I thought that this was the case. However could you tell me where this is stated in BS7671 or in a I&T document so I can confidently re-iterate this point as it has caused many a debate in the past.
Have a look at appendix 15 although informative it specifically states a spur may be supplied from the origin of the circuit. Common sense will also apply in that a spur joined 10cm from the consumer unit is no different than one fed directly from the consumer unit
Q6. So does this mean that it is irrelevant if the final point of the circuit is contained within the distribution board (with the socket/spur bolted onto the side) or away from the board entirely and it also does not matter if the cable is too small for the MCB as long as the cable cannot have its load exceeded?
Another one of the reasons I asked this particular question about cables that run a short distance within an enclosure is because when I look inside busbar chambers there are a very diverse range of cable sizes supplying local isolators which then supply equipment within the installation. Does it not matter if the cable sizes to these isolators is very small when the fuse size within the main switch to the busbar chamber is very large because they are contained within an enclosure?

See the answers in the above quote.
Q6. You have now changed the parameters of your question, the example you gave was OK, now you want something different to also comply based on the original example.
If you read 433.2.2 this specifies where and how a cable can be run without overload protection, but with fault current protection.
Generally it must be <=3m and protected against damage.

 

[IQ Electrical]

Hello all,

I have recently begun carrying out more I&T jobs and have a few questions that I wanted to put to the forum.

1. Why are Zs readings, taken at the main switch disconnectors at distribution boards fed via UPS systems so high and how do these readings satisfy maximum permitted Zs values from their respective MCB?


Cheers for reading all.

I'm a bit late to this but for question 1, the Zs result is meaningless as you are testing through the output (isolated) stage of the UPS.
Either put the UPS into bypass or check with the manufacturer as systems usually have inbuilt fault protection and will disconnect in around 100ms as per EN 50091 and EN62040.

 

[Deleted member 26818]

Information relating to Q5 can be found in appendix 5 of BS7671.