zs and r1+r2

[D Brennan]

Quick question, on a ring main if you have a switched fuse feeding 2 outside lights in 1.5, do you record the zs results you take from the lights cause these are my highest?? also when doin r1+r2 on the same ring main when testing each outlet do i still record the results taken fromm the lights cause again there my highest.

Prob a silly question but ive confused myself.....dont take much:^O

 

[Robojin]

Always take note of the highest Zs and R1+R2 of each circuit in question

Essentially this is the worst case scenario in the event of a fault, so if you can meet your disconnection time at that, point you would expect to meet it on the rest of the circuit, also your Zs compliance should be based on the ring CB not lighting

Other considerations would be that you have an 20A rated cable on a 32A CB so ensure your FSU has an appropriate rating for the fuse for the lighting circuit

Presumably this is on an RCD/RCBO

 

[D Brennan]

Always take note of the highest Zs and R1+R2 of each circuit in questionEssentially this is the worst case scenario in the event of a fault, so if you can meet your disconnection time at that, point you would expect to meet it on the rest of the circuit, also your Zs compliance should be based on the ring CB not lighting

Other considerations would be that you have an 20A rated cable on a 32A CB so ensure your FSU has an appropriate rating for the fuse for the lighting circuit

Presumably this is on an RCD/RCBO

In the fcu a 3a fuse is in place, yes all is rcd protected, i was just getting confused at whether to record the r1+r2 readings and zs readings i get from the lights that are fed from the ring because these are my highest readings.

Am i correct to record these reading on the eic being the highest.

or do i just take the readings i get from socket outlets and fcu's?

 

[Robojin]

In the fcu a 3a fuse is in place, yes all is rcd protected, i was just getting confused at whether to record the r1+r2 readings and zs readings i get from the lights that are fed from the ring because these are my highest readings.Am i correct to record these reading on the eic being the highest.

or do i just take the readings i get from socket outlets and fcu's?

You always record the highest for both Zs and R1+R2 on the circuit, (likely the lamp holder at the end of this spur), in the same way you would if this were a spur for sockets because it is on the same CB, crucially your readings must not exceed 1.16ohms if protected by 32A CB, even with an RCD in place, if you are struggling to meet this you may need to look at the cable length or csa of your lighting spur

By all means take readings at each socket to ensure conformity on the ring, but you would not expect these to be higher and all should be substantially the same, any that appear inconsistent or higher should be investigated even if under the Zs for the CB type

You may already do this, when you crossover in Step 2&3 take a reading at the CU as a reference for the rest of the ring, this will give you an idea of what to expect at each socket, also useful when looking for spurs

Lastly as this circuit is a spur from the ring it would be prudent to label the FSU for these lights e.g. "CU - CBx", I use a dymo 9mm white label for this purpose on all sockets/switches where a label is unobtrusive (ie the customer does not get in a flap about it)

 

[Nicky Tesla]

your f/spur is actually a small one position fuse board, so in fact what should be done is to treat it as such. Ze for the lighting cct is the Zs for the spur, and protection would be from the 1362 3A fuse not the mcb back in the board.

but then again if the readings at the lights are still in tolerance for the mcb, then just write this down to not complicate matters on the cert.

 

[Andy™]

Ze for the lighting cct is the Zs for the spur, and protection would be from the 1362 3A fuse not the mcb back in the board.

Ze is always external impedance. it cannot be measured anywhere other than origin. If you wanted to take a measurement for another DB (or fused spur as is the thread), then it would be Zdb and would be taken within gas and water connected. but for adding a fused spur, all thats needed is R1R2 and Zs

the protection by the 1362 3A fuse would only apply to the circuit after the fused spur, the addition of the fused spur would have its protection as whats in the DB.

And if you really wanted to make the fused spur your DB, then you would need to fill in in EIC rather than MWC, since you would have created a new circuit from your fused spur...

 

[D Brennan]

The fused spur is already on the ring main, its not spured off the ring, the fused spur feeds 2 outside lights, so i was just clarifying that when i do my r1+r2 test and zs test that i actually record the readings as the highest which are took at the outside lights.

Obviously they are the highest because of the run from the fcu but they are within the acceptable results.

 

[Andy™]

you should do your R1R2 from the DB

 

[batty]

You always record the highest for both Zs and R1+R2 on the circuit, (likely the lamp holder at the end of this spur), in the same way you would if this were a spur for sockets because it is on the same CB, crucially your readings must not exceed 1.16ohms if protected by 32A CB, even with an RCD in place, if you are struggling to meet this you may need to look at the cable length or csa of your lighting spurBy all means take readings at each socket to ensure conformity on the ring, but you would not expect these to be higher and all should be substantially the same, any that appear inconsistent or higher should be investigated even if under the Zs for the CB type

You may already do this, when you crossover in Step 2&3 take a reading at the CU as a reference for the rest of the ring, this will give you an idea of what to expect at each socket, also useful when looking for spurs

Lastly as this circuit is a spur from the ring it would be prudent to label the FSU for these lights e.g. "CU - CBx", I use a dymo 9mm white label for this purpose on all sockets/switches where a label is unobtrusive (ie the customer does not get in a flap about it)

Surely a 3amp fuse does not take 1.16ohm to blow so what ever it takes to blow a 3 amp fuse is the important thing not 1.16ohm

Batty

 

[Robojin]

Surely a 3amp fuse does not take 1.16ohm to blow so what ever it takes to blow a 3 amp fuse is the important thing not 1.16ohmBatty

Yes 3A is the key, the Red book does not have a time curve for 1362, but might be around 25A for 0.1s & 10A for 5.0s

Mind you what would be the consideration if someone replaced this later with a 13A cause that's all they had, mark up the switch "3A max" ?

 

[D Brennan]

Right ive read through the thread now and loads of opinions again, when filling out the eic for the ring main...... on tests 2 /3 were you join outgoing live to incoming earth and vice versa..test at each point, highest gives r1+r2 dead simple, bit im getting confused with is when im testing at each point do i also test at the lights that are fed off the ring main via fcu??

next bit is the zs, same again test at each socket outlet and record the highest, as the lights outside are fed from thr ring main via fcu do i record the zs from these as they are the highest reading??

 

[Andy™]

you need to test to each point,so if you have SFCU feeding 2 lights, R1R2 at both lights (which will always be higher than at SFCU, and if there was a problem at SFCU, you wouldnt get reading at light)

 

[D Brennan]

you need to test to each point,so if you have SFCU feeding 2 lights, R1R2 at both lights (which will always be higher than at SFCU, and if there was a problem at SFCU, you wouldnt get reading at light)[/quoteWhen recording the r1+r2 results for the ring main, am i recording the highest result like normal from the ring main outlets or do i record the r1+r2 from the lights?

also do i record the highest zs taken from the ring main outlets or from the lights which are the highest zs.

 

[Nicky Tesla]

Ze is always external impedance. it cannot be measured anywhere other than origin. If you wanted to take a measurement for another DB (or fused spur as is the thread), then it would be Zdb and would be taken within gas and water connected. but for adding a fused spur, all thats needed is R1R2 and Zsthe protection by the 1362 3A fuse would only apply to the circuit after the fused spur, the addition of the fused spur would have its protection as whats in the DB.

And if you really wanted to make the fused spur your DB, then you would need to fill in in EIC rather than MWC, since you would have created a new circuit from your fused spur...

the origin is from where you personally decide to originate from, in this case at the f/spur. normally at the incomer.

In a large installation with many DB's the Ze would be taken at the supply(the distribution cct) to each of the DB's and not at the incomer. The f/spur has a supply which then feeds a final cct

Same Thing

 

[Andy™]

the origin is from where you personally decide to originate from, in this case at the f/spur. normally at the incomer. In a large installation with many DB's the Ze would be taken at the supply(the distribution cct) to each of the DB's and not at the incomer. The f/spur has a supply which then feeds a final cct

Same Thing

the Ze is external to building, so can only be taken at the service head, and is taken in isolation from parallel paths. If you have a submain to a DB, then the submain has its own R1R2 and Zs, this Zs is the Zdb to what it feeds.

 

[green-hornet]

Maximum zs for a 3 amp 1362 fuse is 13.12 @0.4s and 18.56 @5s.

If you was to record the highest zs and r1+r2 on the circuit protected by a 60898 mcb say 32 amp the maximum zs would be 1.15.

I doubt you would come anywhere near these readings if you tested at the furthst light on the circuit.

Always record the highest zs and r1+r2 for any circuit.

The final ring circuit will not be effected by the addition of a fused spur as it is essentially unbroken. I always test with spur off and spur on.

(no cowboy jokes)

 

[Apache]

Maximum zs for a 3 amp 1362 fuse is 13.12 @0.4s and 18.56 @5s.If you was to record the highest zs and r1+r2 on the circuit protected by a 60898 mcb say 32 amp the maximum zs would be 1.15.

I doubt you would come anywhere near these readings if you tested at the furthst light on the circuit.

Always record the highest zs and r1+r2 for any circuit.

The final ring circuit will not be effected by the addition of a fused spur as it is essentially unbroken. I always test with spur off and spur on.

(no cowboy jokes)

 

[D Brennan]

Maximum zs for a 3 amp 1362 fuse is 13.12 @0.4s and 18.56 @5s.If you was to record the highest zs and r1+r2 on the circuit protected by a 60898 mcb say 32 amp the maximum zs would be 1.15.

I doubt you would come anywhere near these readings if you tested at the furthst light on the circuit.

Always record the highest zs and r1+r2 for any circuit.

The final ring circuit will not be effected by the addition of a fused spur as it is essentially unbroken. I always test with spur off and spur on.

(no cowboy jokes)

So on the eic what do u actually record for the r1+r2 and zs, do you just record your higest at each outlet on the ring circuit or do u record from the final on the sfcu

 

[green-hornet]

Record the highest zs and r1+r2 that you get, including the spur.This is the worst case fault you could get.The ring will not be effected by the fault at the lighting circuit and will still perform regardless.

I do know from experiance that circuits will be tested with fused spurs switched off, so they do not include them on the eic, I always do, as a matter of thats the way I do things.I test the whole circuit and put the highest readings I get, I never calculate zs I always test for it. It is surprising to find that often the actual zs is higher than the calculated.

 

[SPECIAL LOCATION]

the Ze is external to building, so can only be taken at the service head, and is taken in isolation from parallel paths. If you have a submain to a DB, then the submain has its own R1R2 and Zs, this Zs is the Zdb to what it feeds.

Ze is NOT external to the building.

It is "External to the installation"

and the Installation can be any

"assembly of associated electrical equipment having co-ordinated characteristics to fulfil a specific purpose."

As such a site could have multiple distribution board which can be considered as individual installations, if you so choose.

Whilst 'Zbd' is a "convention" that some 'contractors' and 'approval schemes operators' like to use to identify the Ze of a particular distribution board (or CU)

In strict sense Zdb does NOT exist within BS7671..

as in Part 2 Definitions, Page 36, we find...

Z

Z1

Ze

Zk

Zm

Zs

Zx

and..

Z