Insulation resistance test? how does it work

[LukeScotty]

Trying to understand how the insulation resistance test works
this is how I think it works, i maybe right or maybe wrong?????????

bascially, your shorting circuiting neutrel and phase.
Sending 500v down the cable (If no voltage no current)
So the tester sends out a current, then reads the return Current and calculates the leakage

then using ohms law, calculates the resistance

and this is a DC test?


is this right?

regards luke

 

[ian.settle1]

Luke

check you answers on IET website

 

[TonyM58]

luke,

erm....basically yeah

Its applying a DC voltage accross two points and measuring the resistance to current flow between them

the tester doesnt 'send out' a current. Current is drawn by whatever circuit the voltage is applied to

and you are not short circuiting neutral and phase. If you mean joining the neutral and phase together at the tails and checking between them and earth, then that is just one method of carrying out an IR test

 

[LukeScotty]

on the test meter, is it just one lead which sends out voltage? and the other lead monitors whether it recieves a current of sum sort?

 

[TonyM58]

Luke, no basically!

voltage (potential difference) has to have 2 points (for there to be a difference in potential!!)

if testing at 500V (for example) then 500V DC is applied to the end of the probes (i meen 500v between the two probes) and the meter will register how much resistance to current flow it encounters

 

[LukeScotty]

cheers, sounds good now, understand it, but dont ask me any questions on it hehe

 

[TonyM58]

oh but i will Luke, i love questions

ok, classic 2391 question

if testing between the input and output phases of a selv circuit, what is the minimum insualtion resistance value (16th edition answers please)

and 10 circuits, each having resistance phase to earth of 10megohm, what insualtion resistance would be measured at the tails?

off you go then.....

 

[gally]

0.5Megohm and 1 Megohm - Sorry, I love (trying) to answer

 

[Grae79]

0.25 Megohms for SELV surely, Shirley

 

[Richard]

I would have said 0.5Mohm.. if under 2Mohm, it should be investigated, so i`m told.

 

[ian.settle1]

Table 71A Reg 713-04-03

SELV & PELV - 250volt 0.25Mohms

Upto & including 500 V - 500v 0.5 Mohms

Above 500v - 1000v 1.0 Mohms

 

[TonyM58]

Well done Gally

come on Ian i tought you knew the book!!!

Yes if it was SELV it would be 0.25V,

but i asked about checking between the input and output phases of a SELV circuit. In this case you work on the highest votage involved, which is the 230V supply.

So you would test at 500v with 0.5M as the minimum

I can find the reg number if you wish, but aint got the book in front of me...........

back of the class, hands on head, no talking.

 

[LukeScotty]

its questions like these which I need to study, got any more on 2391 testing?

 

[TonyM58]

oh i got plenty Luke,

did you work out the circuits in parallel one? thats very typical as well


ok, another one

a circuit wired in 4mm twin and earth is 10m long. Insulation resistance between live conductors is 10Mohm. R1+R2 is 0.75ohms. If the circuit was to double in length, what be the effect on insualtion resistance value and R1+R2 reading.

and another:

Ring main is wired in 2.5/1.5mm twin and earth. rn is measured at 1 ohm.

what is

r1 value
r2 value
R1+R2 value

and explain simply how you measure the R1+R2 value.


ok, off you go........

 

[spark101]

Luke, no basically!

voltage (potential difference) has to have 2 points (for there to be a difference in potential!!)

if testing at 500V (for example) then 500V DC is applied to the end of the probes (i meen 500v between the two probes) and the meter will register how much resistance to current flow it encounters

I don't understand how an IR tester running on 6 AA batteries can shoot 500v??