2391 verfying volt drop

[mk1tina]

Hi Guys n Girls
Sorry if im having a dumb momment but I have a practical 2391 exam in march and i cannot remember for certain how to verify Vd.

Im assuming i can calculate the circuit length by taking the r1+r2 reading - then get the m'ohms per/m for both cables from the (OSG P166). Then divide the measured reading of say 0.3 by the milli ohms per metre to get the circuit length.

So do i then just do a simple Volt drop as per P124OSG using cable charts P274 in BS7671, my main concern is how about a 3phase Vd? You may not b surprised to hear i missed the Cable calc bit out the written exam... I passed it though lol

Just tell me what im missing, im not proud, after all I knew it all once and since forgot it all lol

Thanks
Mark the Spark

 

[scotsparky]

you gont have tyo verify volt drop in the practical however i do recoment you study the periodic test sheets.

 

[mk1tina]

you gont have tyo verify volt drop in the practical however i do recoment you study the periodic test sheets.

Hmmm. Ive been flicking throught the books and eventually things starting to come back online with the old brain!. I tried a few calcs and it all looks to be adding up at last..
As for the forms, thats this weekends browsing material LOL
I hear their wil be a motor circuit on the board, Im assuming u measure the IR between cpc and PN linked as the motor control will have a coil in it???

Thanks
Mark

 

[scotsparky]

they use the IET forms from the BRB and it will probbably be the periodic ones. pay attention to page 3 the tick sheets that is important.

 

[WayneL]

You can always measure it

I didn't have to actually verify the volt drop in mine - the assessor

just asked me to describe how I'd do it.

I just said by measurement or by calculation (the way you described)

except you'ld need the circuit length to do the OSG calculations - could

be a bit tricky on a 2391 board because the circuits contain resistors.

Failing that, find the resistance of the cable, like you said (line and neutral)

and use ohms law with the design current.

Eg.

Lighting circuit:

R1 - 0.6 ohms

Rn - 0.5 ohms

Total - 1.1 ohms

Volt drop = 6 A x 1.1 ohms = 6.6 V

I think

R2

 

[Des 56]

You can always measure it


Eg.

Lighting circuit:

R1 - 0.6 ohms

Rn - 0.5 ohms

Total - 1.1 ohms

Volt drop = 6 A x 1.1 ohms = 6.6 V

I think

R2

But that assumes a 6 amp load

 

[scotsparky]

volt drop is calculated using the desighn current not the rating of the overcurrent device. to truly calaculate it you have to have the true lenght of run, desighn current asw well as cable resistance which is why its frequently limited in a periodic

 

[WayneL]

But that assumes a 6 amp load

Well I used 6A as an example of the design current - I did say:

. Failing that, find the resistance of the cable, like you said (line and neutral)

and use ohms law with the design current.


Like I said though, easiest way is to measure it.

 

[scotsparky]

you could measure it by taking incoming voltage at source then measuring voltage at end of line. it has to be under load though.

 

[ezzzekiel]

you could measure it by taking incoming voltage at source then measuring voltage at end of line. it has to be under load though.

like he said

 

[WayneL]

you could measure it by taking incoming voltage at source then measuring voltage at end of line. it has to be under load though.

Thanks, Scotsparky, for reminding me of that fact

I was thinking you could just measure at the origin and then measure at

the end of the circuit - forgetting that the load counted.

In the 2391 this probably wouldn't be the simplest method then, as most

of the circuits don't have loads attached.

 

[mk1tina]

here here lol.
many thx for the advice