Exam question

Siholtye · Jan 6, 2019

Question 48 on this mock paper I have asks,

A 3-Core, 2 Metre flex, has a calculated earth bond reading of 0.152 Ohms. Which nominal conductor c.s.a would this relate to:
A 0.75mm²
B 1.00mm²
C 1.25mm²
D 1.5mm²

What is the answer, how do you calculate it, and where in the COP is this info please?

SparkyChick · Jan 6, 2019

I'm going to dive straight in... welcome to the forums

How do you think you would calculate it?

Ian1981 · Jan 6, 2019

Its a terrible worded question!
Calculated earth bond for 2 meter flex, who the hell wrote that?

Richard Burns · Jan 6, 2019

Welcome to the forum.
The question appears to mix up two different disciplines: fixed wire testing and in service inspection and testing.
I am assuming the question is about in service inspection and testing since you mention the COP.
Appendix VI contains the resistances of flexible cords, which would be your reference for the answer.

However the answer does not match with the method of testing the "earth bond" as the measurement should only encompass the circuit protective conductor of the flex, yet the question appears to be assuming an R1+R2 value for the calculated "earth bond".

westward10 · Jan 6, 2019

The CoP has this Table for cable resistance in the Appendices but unless I am misreading their question is incorrect.

Wilko · Jan 6, 2019

Yes an error I reckon too ...
Just for fun - my bet is it's 2m of 0.75mm which is 0.052 Ohms (from Table VI.1 in 4th ed)

SparkyChick · Jan 6, 2019

I was wondering whether the figure they provided was actually an R1+R2 value because 0.152/4 (2m there and back) = 0.038... 0.001 ohms off the figure for 1m of single 0.5mm

Wilko · Jan 6, 2019

Interesting thought SC, but 0.5mm isn't one of the answer options

.

westward10 · Jan 6, 2019

PAtesting generally only refers to the cpc reading as it is the only prescribed continuity test.

SparkyChick · Jan 6, 2019

Wilko said:
Interesting thought SC, but 0.5mm isn't one of the answer options .

Good point

patdev · Jan 6, 2019

Wierdly worded question but I doesn't the 0.152 value given include 0.1 Ohms for the appliance?
(not that they mentioned any appliance)

2 Metres at 0.75 mm2 gives a resistance value of 0.052 Ohms

westward10 · Jan 6, 2019

patdev said:
Wierdly worded question but I doesn't the 0.152 value given include 0.1 Ohms for the appliance?
(not that they mentioned any appliance)

2 Metres at 0.75 mm2 gives a resistance value of 0.052 Ohms

I do believe you have solved the problem we have overlooked the 0.10 ohm to be added to the cable resistance.

Richard Burns · Jan 7, 2019

I think patdev has it.
It would have been just so easy for the question to state "calculated maximum permissible earth bond reading of 0.152Ω"
A calculated reading is what reading you would expect to see on the meter when you test and not the maximum reading you could get to allow the tested equipment to pass.
I am obviously still learning the language of C&G speak !