R1 + R2 readings

[davidjones89]

if someone could just explain this please, when conducting R1 + R2 tests on a radial lighting circuit and say the highest reading you get is 0.82 ohms, where do you cross reference the readings ? ie what would be to high a reading

 

[Sparky Ninja]

You could either use the simple formula of:

ZS=(R1&R2)+ZE

or cross reference it with Table A.1 from Guidance Notes 3

 

[JUD]

A measurement of R1 + R2 is mainly used to prove continuity of protective conductors. However the readings you get from this test can be used to calculate the Earth Fault Loop Impedance (Zs) by adding it to Ze.

Zs = Ze + (R1 + R2)

The maximum Zs values are found on pages 48 - 50 of BS 7671 (Tables 41.2, 41.3, 41.4 and 41.5). The values in these tables should be multiplied by 0.8 (rule of thumb) and the measured or calculated Zs should not exceed this corrected value.

A reading of 0.82Ω on a 6A lighting circuit is quite low. If it is wired in 1.5/1mm² then from the conductor resistance table on page 166 OSG the length of the circuit is only 27m [0.82 ÷ (30.2 ÷ 1000) = 27m]. The maximum length for a 6A (Type B) lighting circuit is 90m (limited by voltage drop) from Table 7.1 OSG giving you a maximum measured R1 + R2 of 2.71Ω [90 x (30.2 ÷ 1000) = 2.71Ω] and the maximum Zs is 6.13Ω so you're ok.

Hope that helps.

 

[nickblake]

that depends on the length of the circuit , you can calculate the measurement by the resistance of the cable x length , but not easy to do in a house thats wired just esti mate but what you could do is take a Ze reading then a Zs reading and take one from the other and this should give you your R1 and R2 , but normally anything over 2 ohms should be looked at as a high reading ,thats a level i set for my self not in the regs , over the years you get used to what normal readings should be , and anything below that should be ok and if the readings of the Zs are ok its fine

 

[oldtimer]

I think we have had this raised before but if you have a Ze and a Zs then surely you can get R1+R2 by calculation In other words R1+R2 = Zs - Ze

 

[Sparky Ninja]

I think we have had this raised before but if you have a Ze and a Zs then surely you can get R1+R2 by calculation In other words R1+R2 = Zs - Ze

You can do that quite simply.


The only slight concern is that measuring a Zs is il-advised on some circuits where it can contravene the electricity at work regs. (Working live) - This is the opinion of the NICEIC as far as I am aware.

I myself, measure Zs as you have suggested.

 

[JUD]

that depends on the length of the circuit , you can calculate the measurement by the resistance of the cable x length , but not easy to do in a house thats wired just esti mate but what you could do is take a Ze reading then a Zs reading and take one from the other and this should give you your R1 and R2 , but normally anything over 2 ohms should be looked at as a high reading ,thats a level i set for my self not in the regs , over the years you get used to what normal readings should be , and anything below that should be ok and if the readings of the Zs are ok its fine

The problem with doing it this way is that when you measure Zs you do so with bonding in place which give you parallel paths to earth and more than likely a lower reading than the sum of Ze + (R1+R2).

So as Ze is measured with bonding disconnected simply taking Ze from Zs will not give you a true R1+R2.

 

[oldtimer]

Jud
I hear what you say about a true reading but would point out that this will always be the case with parallel paths as the procedure is quite specific you only disconnect the main earth to take the true Ze reading. I take it this was taken into account when the procedures were written up

 

[JUD]

I'm sorry, I'm not with you. Which procedures are we talking about.

A measurement of Zs with parallel paths will give you a lower reading than calculating it (Zs = Ze + (R1+R2)) and therefore cannot be relied upon to calculate R1+R2 by taking away Ze as this would in turn give you a lower R1+R2 than you would get by measuring it in the normal way.

 

[Sparky Ninja]

As JUD says, the measurment of true Ze will be different to the value of Ze measured in the Zs reading as parallel paths will have been re-introduced in order to carry out the Zs test.

Thats a mouthful.

This is why when you truly take the measurments you will find that 9 times out of 10, R1+R2 is never equal to Zs-Ze

 

[john h owen]

test sheet result indicate that the Zs of the circuit is less than the sum of Zs = Ze +(R1+R2)
Give three reasons why?

 

[Sparky Ninja]

Is this a question?

 

[jeremy]

I came up with 2 but it's been a long day. Parallel paths and if measuring Zs on a socket with a plug connector the contact may be more reliable than probe points. please give me the answer

 

[pushrod]

I came up with 2 but it's been a long day. Parallel paths and if measuring Zs on a socket with a plug connector the contact may be more reliable than probe points. please give me the answer

Could it be something silly like test leads not nulled when doing R1+ R2 ?

 

[Sparky Ninja]

I can think of 50 possible reasons.