2391 one for the anoraks.

[La Poste]

The testing on a new circuit provided the following results.

Ze= 0.3 Ohms

R1+R2 = 1.4 Ohms.

Max value in BS7671 listed for the circuit = 2.1 Ohms.

a) Calculate the expected measured value of Zs.
b) determine whether this circuit will comply with BS7671

a) Zs = 0.3 + 1.4 = 1.7 Ohms
b) 2.1 times 0.8 (rule of thumb) = 1.68 Ohms.
The circuit will not comply.

Or.

a) Zs = 0.3 + 1.4 = 1.7 Ohms.
b) Using the other rule of thumb.
Zs = 0.3 + (1.4*1.2)
1.4 Times 1.2 = 1.68 Ohms.= R1+R2
Zs = 0.3 + 1.68= 1.98 Ohms.
The circuit will comply.

Depends which rule of thumb they want applied just to confuse matters.
Personally I would use the 0.8 as that's what I think they are asking.

 

[JUD]

The testing on a new circuit provided the following results.

Ze= 0.3 Ohms

R1+R2 = 1.4 Ohms.

Max value in BS7671 listed for the circuit = 2.1 Ohms.

a) Calculate the expected measured value of Zs. 1.7Ω
b) determine whether this circuit will comply with BS7671. No. Max Zs @ 70°C is 1.68Ω (2.1 x 0.8 = 1.68)

a) Zs = 0.3 + 1.4 = 1.7 Ohms
b) 2.1 times 0.8 (rule of thumb) = 1.68 Ohms.
The circuit will not comply.

Or.

a) Zs = 0.3 + 1.4 = 1.7 Ohms.
b) Using the other rule of thumb.
Zs = 0.3 + (1.4*1.2)
1.4 Times 1.2 = 1.68 Ohms.= R1+R2
Zs = 0.3 + 1.68= 1.98 Ohms.
The circuit will comply.

Depends which rule of thumb they want applied just to confuse matters.
Personally I would use the 0.8 as that's what I think they are asking.

I'd stick with the 0.8 figure as this is what the regs advise you to use when calculating Zs although it also says a more accurate method (the other way you've done it) can be used (Appendix 14).

 

[tony mc]

I would also stick to the rule of thumb method when testing (0.8) as this takes into account the increase of resistance of the conductors and the increase of temp due to the load current also its errs on the side of safety.

 

[Dunc]

a) Calculate the expected measured value of Zs.

1.7Ω

b) determine whether this circuit will comply with BS7671. No. Max Zs @ 70°C is 1.68Ω (2.1 x 0.8 = 1.68)

Max Zs listed in BS7671 is for a circuit running at 70°C, 0.8x max Zs relates to circuits operating at an ambient temperature of 30°C as fair as i know. Someone correct me if i'm wrong but why else would you have two values.

 

[JUD]

0.8 is a rule of thumb.

The most accurate way of calculating it is to measure the ambient temperature at the time of the test, and adjust the value you get from the Zs measurement down to 20°C before then adjusting it to the cable operating temperature (70°C).

Example

Ambient temperature -- 25°C

Measured Zs -- 0.86Ω

From table 9B in the OSG the correction factor for 25°C is 1.02 (or 1 + (0.004 x difference in temperature from 20°C) -- 1 + (0.004 x 5) = 1.02) so you divide the measured Zs reading by 1.02 to bring it down to 20°C.

0.86 ÷ 1.02 = 0.84Ω

You then adjust the new value to the conductor operating temperature of 70°C.

Table 9C in the OSG gives you the correction factor for 70°C as 1.2 (or 1 + (0.004 x difference in temperature from 20°C) -- 1 + (0.004 x 50) = 1.2) so as you're going up in temperature, this time you multiply the value by 1.2.

0.84 x 1.2 = 1Ω

This is the Zs reading you would compare with the tables in BS 7671:2008 and as long as your corrected value is lower, then the circuit will comply