Ring final circuit conductors and value Zs and R1+R2

[sparkyspark2258]

Hi all,
I would like to ask you about ring final circuit canductors and schedule of test results.
I have got dedicated ring circuit to supply kitchen appliances. Circuit is protected by mcb tybe B 32A. Then supply is split into radial circuits to supply appliances by switched 13A FCUs. What value should be recorded in schedule of test results as maximum permited Zs? Is it value for mcb- 1.37 or value for fuse 1362 13A- 2.30 ?
Also what value need to be recorded in R1+R2 value? Is it (r1+r2)/4 or measured value of R1+R2 at last point in circuit (single socket to supply appliance)?

Thank you for any help.

 

[STM]

Hi mate,

You would record 1.37 as your maximum Zs value.

And for your R1+R2 value, this would be determined by doing your RFC 'figure of 8' test. You could verify your measured value is as expected by using the r1+r2/4 calculation.

Cheers

 

[Richard Burns]

I would agree that the Zs value should be the 1.37Ω for the 32A MCB.
It would be possible to have a spur that is protected by ADS by the 13A fuse however it is better to have the whole circuit protected by the MCB at origin.

In doing the figure 8 measurements when you measure the resistance at the end of one of the spurs you are measuring the resistance of the spur back to the ring plus the figure 8 expected [(r1+r2)/4] resistance in series, so the value may be higher than the (r1+r2)/4 calculation, however this is OK because you know it is a spur (so long as the resistance is compliant!) .
Therefore you can obtain the R1+R2 reading to record as the highest reading taken whilst doing the line to cpc crossover measurements at each point of use.

 

[sparkyspark2258]

Thanks for that.

I would like to ask also for testing of 2-way and intermediate switching.
In scenario we have got:
2-way, intermediate, intermediate, 2-way.

Let`s start from R1+R2 value.
Test probes on last light fitting point in circuit on L and cpc.
Taking a reading, then
switching first swich to opposit position, reading
then second switch to opposit position,reading
then third swich to opposit position, reading
then fourth switch to opposit position, reading
then third swich to opposit position, reading
then second switch to opposit position,reading
switching first swich to opposit position, reading

So those tests verify polarity also.

Leave switch in on position.

Then IR test in sequence:

Between L and E
take a reading
Between L and N
take a reading
Between N and E
take a reading

then switch first switch to opposite position and repeat the test:

Between L and E
take a reading
Between L and N
take a reading
Between N and E
take a reading

then switch second switch to opposite position and repeat the test:

Between L and E
take a reading
Between L and N
take a reading
Between N and E
take a reading

then switch third switch to opposite position and repeat the test:

Between L and E
take a reading
Between L and N
take a reading
Between N and E
take a reading


then switch fourth switch to opposite position and repeat the test:

Between L and E
take a reading
Between L and N
take a reading
Between N and E
take a reading

Is that correct?

 

[Richard Burns]

Slightly more complex to answer this one as there is a practical approach and an approved testing approach!

In testing R1+R2 any reading on a light circuit, where the light would be on if the circuit were energised, should be substantially the same and all readings where the light would be off should be open circuit. However to test for switching in the line conductor each switch should be operated to verify this.
Therefore your sequence of testing for R1+R2 would be correct except that the last action you show would duplicate the first, so is not necessary. The readings would be taken to verify disconnection of the line when a switch is off.
Practically if there are two people; testing when switch one is in one orientation and then the other, then the same for switch two, and three, and four might be easier.

For IR testing the light switch is only switching the line and so the status of the Neutral and Earth do not change so there is only a necessity to test the Neutral to Earth insulation resistance once.

The testing of insulation resistance needs to test the line against the neutral and the earth, but the switched line will vary depending on how the switches are positioned (for two wire/conduit wiring).
Your sequence would correctly test all the line conductors.
This diagram of two wire wiring shows the options you have given and the IR test from L to N and L to E would be testing the red coloured conductors (those that would be live when energised).

On a practical basis, not when doing exams, note that you would have covered all conductors after the first change.
Also if you are doing initial verification and know which switch is which, then for three wire / conversion wiring when the light would be on all line conductors would be tested so you can test L to N and L to E and then reverse one switch and test L to N and L to E again and be sure of covering the options.
For two wire / conversion wiring testing L to E and L to N and then reversing the position of the first and last switches (the two way switches) and testing L to E and L to N would cover all the line conductors.

 

[Pete999]

Are you saying you have a ring or two radials, misleading to say the least, if I am reading it wrong apologies

 

[Richard Burns]

Are you saying you have a ring or two radials, misleading to say the least, if I am reading it wrong apologies

I think they do not mean radials, they mean switched fused spurs from the ring, just a nomenclature issue.

 

[sparkyspark2258]

.

 

[sparkyspark2258]

Thank You for fantastic reply Richard Burns.

Is it prefered method to mesure R1+R2 by regs? Sequence from my first post or suggested by Richard when two people available.

Regarding IR test, just to be sure:

-option three wires, as second part of this test, only one switch needs to be operated
-option two wires, as second part of this test, both two way switches needs to be operated

 

[Richard Burns]

The on site guide only gives the advice that on insulation resistance testing that the switches must be operated one at a time and further measurements taken (not very specific!). Your training provider should advise you on their approved method for you to use in training.

When actually doing testing use the most convenient method available at the time that will allow you to be sure of your results.
R1+R2 should be, for initial verification, measured at each point in the circuit, and so when measuring at switches the switches can be operated on and off at the time to check the line is being switched.

As a short answer on the IR testing, YES.
As a long answer:
The attached diagram which is an old one shows diagrammatically the cables that are live on a three way switched circuit for both two and three wire wiring.
You will be measuring IR from the origin of the circuit so this effectively applies the test voltage to the incoming permanent line for the switches, therefore you will test all the red coloured conductors in any one diagram when you test, any that are brown will be omitted from the test and the switches have to be set so that these brown ones then become red.
If you look at three wire wiring you will see that all conductors are connected to incoming live when the light would be on so in that case an IR test (L-N and L-E) will cover all line conductors in the switching arrangement.
For two wire wiring when the switch is on you will cover half the strappers between switches, changing the switch that is supplied by the permanent line will exclude the switched line but switch over to the other strappers, then changing the switch that is connected to the switched line will include the switched line as well as the other strappers.

 

[sparkyspark2258]

Thank You Richard

 

[sparkyspark2258]

I would like to come back to question regarding R1+R2 on ring final circuit and radial circuit from fcu to single socket outlet. What if used fuse is 3A rating and maximum permited Zs for that fuse is 15,6 . What if csa of radial circuit is 1.0mm and mesured Zs is way above permited for 32A B type mcb 1,37 but below 15,6? So ADS in not provided for mcb but is provided for fuse 1362. Just to confirm, is it ok?

 

[sparkyspark2258]

And I would like to ask another question:
what is appropriate name of conductor connected to steel wire of swa cable (flying lead)? Is is earthing conductor, protective conductor or cpc? I have used two armoured cables: one number of swa cable is used to supply submain and second swa is used as supply to metaclad dso.

 

[Richard Burns]

I would like to come back to question regarding R1+R2 on ring final circuit and radial circuit from fcu to single socket outlet. What if used fuse is 3A rating and maximum permited Zs for that fuse is 15,6 . What if csa of radial circuit is 1.0mm and mesured Zs is way above permited for 32A B type mcb 1,37 but below 15,6? So ADS in not provided for mcb but is provided for fuse 1362. Just to confirm, is it ok?

The point on the ring where the fuse is positioned must meet the ADS requirements for the ring.
After the fuse it is preferable but not required to continue the protection from the ring protective device. However if the resistance of the spur after the fuse exceeds the maximum Zs required for the B32MCB but is lower than the maximum Zs required for the BS1362 fuse then that spur is sufficiently protected to be compliant with the requirements of the regs.

And I would like to ask another question:
what is appropriate name of conductor connected to steel wire of swa cable (flying lead)? Is is earthing conductor, protective conductor or cpc? I have used two armoured cables: one number of swa cable is used to supply sub main and second swa is used as supply to metal clad dso.

The fly lead connecting the gland holding the armour of the SWA is a protective conductor.
The SWA armour, if not using the armour as the cpc for the circuit, still needs to be earthed to ensure disconnection in case of cable penetration.
If the SWA armour is being used as the cpc for the circuit then the fly lead is maintaining the continuity of the cpc (and should be sized appropriately) and is part of the cpc for that circuit.

 

[sparkyspark2258]

Thanks, Richard....
I am trying to label conductors and my scenario is:

Main earthing terminal ( next to service head):
-incoming eathing conductor(?) from DNO- TN-s system
-outgoing earthing conductor(?) single core to next earthing terminal next to DB (DB located in different location than service head)
-outgoing earthing conductor(?) - earhing to swa (supply cable to DB by isolator) steel wire and also one of 3-core swa conductor

Second earthing terminal (next to DB):
-earthing conductor(?) from main earthing terminal - single core
-earthing conductor(?) from main earthing terminal - swa steel wire
-earthing conductor(?) from main earthing terminal - swa one of 3-core conductor
-outgoing earhing conductor(?) to DB earthing bar
-main bonding conductor to gas
-main bonding conductor to water
-main bonding conductor to heating pipes
-protective conductor(?) - flying lead to swa supply DB1
-protective conductor(?) - flying lead to swa supply DB2
-protective conductor(?) - flying lead to swa supply metalcad DSO (separate CPC used)
-protecive conductor(?) - flying lead to SY cable gland supply metalcad DSO (separate CPC used)

Hope this have a sense
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