How did i fail on zs in am2?

[SamRimmer98]

Hi all, I done my am2 and passed everything except testing and inspect, i failed on Zs (earth fault loop impedence). It says I did not correctly obtain the value in accordance to guidance note 3, but I did zs=ze+r1+r2. Just wondering if anyone knows where I could have gone wrong?.

Thanks.

 

[Julie.]

Difficult to say,

Was it a ring and you used the end to end instead of the mid point value?

Did you measure ze correctly - i.e. left the earths in place before measuring?

When measuring the circuit r1+r2 did you leave the earth connected to the earth bar?

Did you use the value of max Zs from the regs and not take 80% to account for temperature?

Did you just make a maths error?

Complete in the wrong area of the EIC ?

Lots of possibilities.

 

[SamRimmer98]

Difficult to say,

Was it a ring and you used the end to end instead of the mid point value?

Did you measure ze correctly - i.e. left the earths in place before measuring?

When measuring the circuit r1+r2 did you leave the earth connected to the earth bar?

Did you use the value of max Zs from the regs and not take 80% to account for temperature?

Did you just make a maths error?

Complete in the wrong area of the EIC ?

Lots of possibilities.

Hi Julie thank you for the reply. I took my max zs from the regs, I thought the 80% only applies to values taken from the on site guide.

Thank you, Sam.

 

[telectrix]

Hi Julie thank you for the reply. I took my max zs from the regs, I thought the 80% only applies to values taken from the on site guide.

Thank you, Sam.

the 80% is to allow for resistance value increasing with temp.i.e you measure at 20deg. centipedes, but need to allow for 70deg. which is the rated max. temp.for pvc cable/s. ( critical in Scouseland as most od the copper cables we handle are hot.)

 

[buzzlightyear]

me ,you should have taken from the big blue book ,and not the 80% OSG,it was not long ago the big wigs sent out the wrong dater in the yellow OSG.

 

[pc1966]

I thought the 80% only applies to values taken from the on site guide.

The regs give the value to be met without allowing for temperature, as you might have all sorts of cables in use in strange cases (e.g. very hot cables in a furnace, etc, or practically no Zs derating if cables very over-rated for long run and meeting VD limit, etc) for which you should compute the specific temperature factor to use.

The OSG covers the vast majority of real-life cases when it is specified at 70C PVC limit and makes your life easier and less error-prone by tabulating the values with the 80% factor applied ready to use.

 

[HappyHippyDad]

Isn't the terminology incorrect?

Zs = Ze + (R1+R2) {not r1+r2}

r1 and r2 are the end to end resistances on a ring, which is different to R1 and R2.

 

[Julie.]

Isn't the terminology incorrect?

Zs = Ze + (R1+R2) {not r1+r2}

r1 and r2 are the end to end resistances on a ring, which is different to R1 and R2.

Yes, but I doubt that would be sufficient to fail, it would be something important such as the wrong value etc.

 

[Julie.]

Hi Julie thank you for the reply. I took my max zs from the regs, I thought the 80% only applies to values taken from the on site guide.

Thank you, Sam.

The regs give design values, that is the max allowable value for a 32A type B mcb would be 1.37 ohm, and the resistance of the cable would be as it is when it hot.

When you are commissioning the installation etc the cables would be cold.

So you either use the 1.37 ohm value, and estimate the hot cable from the cold (as tested) cable resistance by multiplying by 1.2.
Or
Use 80% (1.1 ohm) for the max allowable and the cold resistances as is.

The osg assumes the latter and gives the 80% values for the max allowable, and cold resistances within it's tables.

That is why the figures in the regs are different from the osg.

It is easy to get mixed up

 

[HappyHippyDad]

Yes, but I doubt that would be sufficient to fail, it would be something important such as the wrong value etc.

I thought perhaps he had put the wrong values in. He's said he's put in r1 and r2, which would give an incorrect figure.
In fact, there can be quite a bit of confusion when initially learning about r1 and r2 as opposed to R1+R2. This may be where he went wrong.

 

[Julie.]

I thought perhaps he had put the wrong values in. He's said he's put in r1 and r2, which would give an incorrect figure.
In fact, there can be quite a bit of confusion when initially learning about r1 and r2 as opposed to R1+R2. This may be where he went wrong.

Ahh I see, yeah that was one of the options I had suggested - end to end rather than calculated R1+R2.

Very well could have been, sorry I thought you just meant using lower case letters in error.