Max Zs values at different ambient test temperatures

[ppelec100]

Hi guys, I'm currently revising for my 2391 and kind of confused on the temperature factors for the Zs values

The BBB give Max Zs which are 100% values with a max operating temperature of 70c.
The OSG and GN3 give values to 80% which are when tested at ambient temp of 10c according to page 146 OSG. These values are the same if you x 0.8 of the BBB values.
There are correction factors on tables B8 and I2 on OSG

Q1. When using the 80% values which have ambient air temp of 10c do we have to apply the correction factor taken from table I2 OSG page 219 to the cable being tested? For example if I got a reading of say 0.56ohm on R1+R2 would I now need to divide that by 0.96 to reach the ambient temperature of 10c which will now be 0.58ohm R1+R2...

Under table I2 it says the rating factor may be applied in table I2 to the values to take into account of the ambient temp (for test purposes only)

Q2. Also, realistically the ambient temp is not always going to be 10c so lets says if it is 20c when testing circuits, would you then multiply the Zs of In x 1.04? So lets use B32 being 1.1ohm x 1.04 (correction) = 1.14ohm which will now be the Max permitted Zs for B32?

Q3. If the ambient testing temperature is 25c do we then apply a correction factor to both the protective device which will increase its max permitted Zs value by x1.06 as well as the measured R1+R2 readings which will decrease by 2% (1.02)?

So to break my questions down as simply as I can
Q1: if the testing ambient temperature is 10c do we apply the correction factors from table I2 page 219 OSG to the measured R1+R2 to meet the 10c test temperatures set out in B8 page 146 OSG
Q2: if the test temperature is higher or lower than 10c do we then multiply the 80% Max Zs values by the correction factors in table B8 page 146 OSG to meet the ambient temperature at the time of testing?
Q3. If the ambient testing temperature is 25c do we then apply correction factors to both protective device max Zs and R1+R2 readings?

Thank you to whoever takes the time to read and answer my questions!

 

[Pretty Mouth]

Q1: if the testing ambient temperature is 10c do we apply the correction factors from table I2 page 219 OSG to the measured R1+R2 to meet the 10c test temperatures set out in B8 page 146 OSG

Yes

Q2: if the test temperature is higher or lower than 10c do we then multiply the 80% Max Zs values by the correction factors in table B8 page 146 OSG to meet the ambient temperature at the time of testing?

Yes

Q3. If the ambient testing temperature is 25c do we then apply correction factors to both protective device max Zs and R1+R2 readings?

Yes

 

[ppelec100]

Yes

Yes

Yes

Thanks for your reply. So in reality we are supposed to do this? I haven’t seen anyone do it or even mention it before to be honest

 

[ppelec100]

Yes

Yes

Yes

Also, if at the time of testing, if say the ambient temperature is 10c or 25c when measuring the R1+R2 wouldn’t the MFT already measure the conductors at whatever the ambient temperature is so no need to apply correction factors to measured R1+R2?

 

[Pretty Mouth]

Sorry it wasn't a very full answer earlier, I didn't have time to go into greater detail. Also sorry, I have mislead in places with my hasty replies. I'll waffle a bit now, most of which I think you already understand anyway.

The BBB gives max permitted Zs for the protective device. We need to make sure that this would disconnect within the specified time under the most onerous conditions. This would be when the cables are at their full operating temperature (typically 70 deg C), when the conductor resistance is as high as it can be under any normal operating conditions.

In practice, we'll be measuring actual Zs (whether it be by direct measurement, or by measuring the resistance of the cables and adding to Ze) at a much lower temperature than 70 deg. So the OSG appendix B gives tables with the max Zs adjusted down to 10 deg C: If the circuit Zs does not exceed this adjusted max permitted Zs when we measure it at 10 deg, then it will not exceed the unadjusted values from the BBB at 70 deg C.

As long as you measure the circuit Zs (by whichever method) at 10 deg C or higher, and it meets the values in OSG app B, then disconnection is assured whatever temperature the circuit may reasonably end up at. This is quick and easy, and most of the time is all that is necessary, and probably explains why people don't usually bother with correction factors.

However, you might measure a circuit Zs that just goes over the max permitted values in the OSG. In this case, you might be able to apply a correction factor to the OSG values to adjust for the actual temperature.

Say ambient temperature was 25 deg, then you can multiply the OSG max Zs value by 1.06, and directly compare your measured Zs to this new max Zs value.

However, suppose you were measuring Zs at a temperature of 0 deg C. At this temperature, the circuit conductors would have a lower resistance than at 10 deg C. We need to compensate for this. Take the max Zs value from the OSG, and multiply it by the correction factor 0.96. This new Max Zs value can be compared directly to the measured Zs.

 

[Pretty Mouth]

Q1. When using the 80% values which have ambient air temp of 10c do we have to apply the correction factor taken from table I2 OSG page 219 to the cable being tested? For example if I got a reading of say 0.56ohm on R1+R2 would I now need to divide that by 0.96 to reach the ambient temperature of 10c which will now be 0.58ohm R1+R2...

Under table I2 it says the rating factor may be applied in table I2 to the values to take into account of the ambient temp (for test purposes only)

Adjust the 80% max Zs value up to the ambient temperature using table, and compare this new value directly to the measured circuit Zs.

 

[Pretty Mouth]

Q2. Also, realistically the ambient temp is not always going to be 10c so lets says if it is 20c when testing circuits, would you then multiply the Zs of In x 1.04? So lets use B32 being 1.1ohm x 1.04 (correction) = 1.14ohm which will now be the Max permitted Zs for B32?

Yes. Note that in the cert, you would still record the max permitted Zs from the BBB, without any correction factors applied.

 

[Pretty Mouth]

Q3. If the ambient testing temperature is 25c do we then apply a correction factor to both the protective device which will increase its max permitted Zs value by x1.06 as well as the measured R1+R2 readings which will decrease by 2% (1.02)?

Also, if at the time of testing, if say the ambient temperature is 10c or 25c when measuring the R1+R2 wouldn’t the MFT already measure the conductors at whatever the ambient temperature is so no need to apply correction factors to measured R1+R2?

My bad, I said yes, should have said no. Apply the correction factor to the max permitted Zs only, to adjust it to the ambient temperature. Then compare your measured Zs directly to this new value.

TBH, it is very rare that I have to think about this in the work I do. Most circuits are RCD protected at 30mA, so comparing Zs is not really necessary for the most part.