ACA mock test question wrong?

BGboardsarethetits · Thursday at 10:34 PM

When I was doing the ACA exam mock test I have been looking at this question and think it’s wrong? A final circuit supplying a 2 kW boiling water tap is wired using 2.5mm2 PVC insulated
copper conductors with a 1.5mm2 circuit protective conductor, and is 15 m long. The
installation has a Ze of 0.65 ohms. Taking in to account the maximum permissible operating
temperature of 70° C with a correction value of 1.2. What will be the value of the measured
Zs for this circuit?
a) 0.87 Ω
b) 0.91 Ω
c) 0.94 Ω
d) 1.16 Ω

Resistance Value R1+R2 is 19.51x15/1000 = 0.292 so add this to Ze and it is 0.292+0.65 = 0.94. Which they say is the right answer but where is the correction factor. Should that not be 0.94x0.8= 0.75.
Please help can’t get my head round it.

PS Electrics · Thursday at 11:02 PM

BGboardsarethetits said:
When I was doing the ACA exam mock test I have been looking at this question and think it’s wrong? A final circuit supplying a 2 kW boiling water tap is wired using 2.5mm2 PVC insulated
copper conductors with a 1.5mm2 circuit protective conductor, and is 15 m long. The
installation has a Ze of 0.65 ohms. Taking in to account the maximum permissible operating
temperature of 70° C with a correction value of 1.2. What will be the value of the measured
Zs for this circuit?
a) 0.87 Ω
b) 0.91 Ω
c) 0.94 Ω
d) 1.16 Ω

Resistance Value R1+R2 is 19.51x15/1000 = 0.292 so add this to Ze and it is 0.292+0.65 = 0.94. Which they say is the right answer but where is the correction factor. Should that not be 0.94x0.8= 0.75.
Please help can’t get my head round it.

I think it is strangely worded question.

Measured Zs would be at ambient temp. The correction factor is applied to take into consideration that the impedance will be greater when the circuit is at operating temp of 70C ie under load.

So since it asks for measured Zs there is no correction factor to apply.

BGboardsarethetits · 2025-05-16T06:47:16+0100

PS Electrics said:
I think it is strangely worded question.

Measured Zs would be at ambient temp. The correction factor is applied to take into consideration that the impedance will be greater when the circuit is at operating temp of 70C ie under load.

So since it asks for measured Zs there is no correction factor to apply.

Yea I get that that it will be at ambient temp but measured Zs(m) is after you have applied the 0.8 rule? Or are they just wanting what you would get if you put your test probes across it? Like you say it’s a strongly worded question almost a trick question.

PS Electrics · 2025-05-16T07:17:47+0100

BGboardsarethetits said:
Yea I get that that it will be at ambient temp but measured Zs(m) is after you have applied the 0.8 rule? Or are they just wanting what you would get if you put your test probes across it? Like you say it’s a strongly worded question almost a trick question.

The 0.8 correction factor is for the Max Zs values in BS7671 because those values are not corrected for ambient temp. The on site guide gives Zs values at 10C and if you really wanted to go to town there is a formula you can use given the actual ambient at time of testing.

BGboardsarethetits · 2025-05-16T10:29:16+0100

PS Electrics said:
The 0.8 correction factor is for the Max Zs values in BS7671 because those values are not corrected for ambient temp. The on site guide gives Zs values at 10C and if you really wanted to go to town there is a formula you can use given the actual ambient at time of testing.

The values in OSG are 80% of the Max Zs in BS7671. But is this not for operating temp not ambient temp is it not? Is that the formula at the bottom of ambient temp table.

PS Electrics · 2025-05-16T15:41:03+0100

I hope that helps.

BGboardsarethetits said:
The values in OSG are 80% of the Max Zs in BS7671. But is this not for operating temp not ambient temp is it not?

The ambient temp is the temp when testing the cable we don't generally test cables under load so they won't be at operating temp they will be at ambient temp.

If we come under the max Zs listed in the OSG at ambient then the cable WILL comply at operating temp.