R1+r2 reading and r1,r2 and rn on ring final

[Abu]

Hi there
Been stuck on this for a while. What is the maximum r1+r2 reading possible on a lighting circuit in a domestic property. With a type b 6amp breaker.
Also if you guys could tell me about r1+r2 and rn Maximum readings for ring final.
Thanks

 

[Octopus]

Zs = Ze + (R1 +R2) - so you know the Ze, then then you can calculate the R1 + R2 value, then look up the rest in the books

As for a ring - what do you understand about the configuration of a ring and how you would test it?

What are you trying to achieve?

 

[Pete999]

Hi there
Been stuck on this for a while. What is the maximum r1+r2 reading possible on a lighting circuit in a domestic property. With a type b 6amp breaker.
Also if you guys could tell me about r1+r2 and rn Maximum readings for ring final.
Thanks

It's R1+R2 r1, r2 and rn are the end to end resistances of the L, N and CPC R1+R2 is the reading obtained by the figure of 8 test

 

[Abu]

It's R1+R2 r1, r2 and rn are the end to end resistances of the L, N and CPC R1+R2 is the reading obtained by the figure of 8 test

Yes i understand and know how to measure it ut was wondering what is the maximum value allowed for the test

 

[Strima]

Your maximum R1+R2 are dependant upon the rating of the protective device and the Ze of the installation.

 

[Octopus]

Yes i understand and know how to measure it but was wondering what is the maximum value allowed for the test

Post #2 applies....

What are you trying to do?

 

[Phil Thompson]

Max Zs table in back of OSG.

 

[Toneyz]

For electrical certification you can only calculate Zs ,Ze and R1 + R2 are measured using test equipment. If you have a Ze then you can have a designed R1 + R2 based upon the chosen installed cable. Therefore designed Zs from that you can choose a protective device from the tables in BS7671.

 

[Ian1981]

Z41 is max zs from bs 7671 i.e. 1.37 ohms for a type b
Ze maybe unknown at this point
Ls is theoretically the max length allowed

 

[Richard Burns]

If you have the On Site guide then in table 7.1 in chapter 7 this lists the maximum length of cable permitted for standard circuits in various installation methods and with different protective devices.
Then you can compare the circuit lengths with the resistance values given in appendix I to give you maximum values for resistance.

For instance a 2.5/1.5 ring final circuit on a 32A MCB with 30mA RCD protection has a maximum length of 106m. (this is limited by voltage drop)
Therefore on 106m your r1 and rn would be 0.00741*106 =0.785Ω
your r2 would be 0.0121*106=1.28Ω

Similarly for a 1.0/1.0 6A lighting circuit the max length is 59m and so R1+R2 would be 0.0362*59=2.14Ω.

 

[Abu]

Measure the ring final circuit readings i got a ze of o.14 at db on tns system i got a reading of 0.55 for r1 +r2 i would like to know what is the maximum reading allowed .

 

[Abu]

If you have the On Site guide then in table 7.1 in chapter 7 this lists the maximum length of cable permitted for standard circuits in various installation methods and with different protective devices.
Then you can compare the circuit lengths with the resistance values given in appendix I to give you maximum values for resistance.

For instance a 2.5/1.5 ring final circuit on a 32A MCB with 30mA RCD protection has a maximum length of 106m. (this is limited by voltage drop)
Therefore on 106m your r1 and rn would be 0.00741*106 =0.785Ω
your r2 would be 0.0121*106=1.28Ω

Similarly for a 1.0/1.0 6A lighting circuit the max length is 59m and so R1+R2 would be 0.0362*59=2.14Ω.

Thank u very much this is exactly what i wanted to know.

 

[telectrix]

Measure the ring final circuit readings i got a ze of o.14 at db on tns system i got a reading of 0.55 for r1 +r2 i would like to know what is the maximum reading allowed .

you mean R1 +R2. note capitals. add |R1+R2 +Ze. that will give you the Zs for the circuit. Zs must be < the max. value in (tables) for the In of the OPCD.

 

[Abu]

Z41 is max zs from bs 7671 i.e. 1.37 ohms for a type b
Ze maybe unknown at this point
Ls is theoretically the max length allowed
View attachment 38489

 

[Abu]

you mean R1 +R2. note capitals. add |R1+R2 +Ze. that will give you the Zs for the circuit. Zs must be < the max. value in (tables) for the In of the OPCD.

I have seen videos where they directly measure the zs through a megger meter. I that reliable or is it better to do the calculatio.

 

[Abu]

 

[Richard Burns]

In design or initial verification of a circuit you do the calculation to ensure it is safe to power up. Once the circuit is powered then measurement of Zs is a standard, fairly reliable and common method of ensuring that your circuit is compliant with the disconnection times required. Measured Zs can be lower than calculated Zs and so the calculation is the worst case scenario used for safety.

 

[Abu]

If you have the On Site guide then in table 7.1 in chapter 7 this lists the maximum length of cable permitted for standard circuits in various installation methods and with different protective devices.
Then you can compare the circuit lengths with the resistance values given in appendix I to give you maximum values for resistance.

For instance a 2.5/1.5 ring final circuit on a 32A MCB with 30mA RCD protection has a maximum length of 106m. (this is limited by voltage drop)
Therefore on 106m your r1 and rn would be 0.00741*106 =0.785Ω
your r2 would be 0.0121*106=1.28Ω

Similarly for a 1.0/1.0 6A lighting circuit the max length is 59m and so R1+R2 would be 0.0362*59=2.14Ω.

Where do u get the value of 0.00741?

 

[Richard Burns]

Where do u get the value of 0.00741?

Appendix I of the on site guide; the resistance of a 2.5mm² conductor is 7.41mΩ/m which is equivalent to 0.00741Ω/m.

 

[Toneyz]

In design or initial verification of a circuit you do the calculation to ensure it is safe to power up. Once the circuit is powered then measurement of Zs is a standard, fairly reliable and common method of ensuring that your circuit is compliant with the disconnection times required. Measured Zs can be lower than calculated Zs and so the calculation is the worst case scenario used for safety.

Zs includes parallel earth paths eg structural metal of the building ,bonding etc. which rightly stated lowers the measured Zs value also if the circuit has a RCD/rcbo the test current used is lowered as not to trip the RCD/rcbo during the test therefore you get a higher Zs .