zs testing

[Jake1579]

Im on an electrical installation course in college and currently on a 3 month trial as an apprentice so thats why im asking zs testing from what i understand in college they teach it you as zs=ze + r1/r2 which i understand but when i go and actually look at zs test myself say on youtube they talk about like for example a ligthing circuit to test it at the switch line and the earth on the celing rose or light switch depending if its awkward to get to but they say test at the last lighting point so my questions are
1. why do you need to test at the last point?
2. does this test take in to account the ze and r1/2?

 

[GMES]

Hi Jake I have put a link for you to apply for the trainee section if you are on a current course.
http://www.electriciansforums.co.uk/apply-gain-access-trainee-only-forum/

Also offer up your thoughts on the 2 questions and the members will be more than happy to guide you along.

 

[Octopus]

what are your thoughts on the (sensible) answers?

 

[Pete999]

Zs = Ze +(R1+R2), the reason you are testing at the furthest point on a circuit is to take account of all the wiring, testing R1+R2 is a dead test so no it wouldn't take into account the Ze https://www.youtube.com/watch?v=PUTn3t65na4 https://www.youtube.com/watch?v=S-C8cwSA8Vg Hope these vids help

 

[trev]

If Ze + (r1+r2) = Zs then why would a live Zs test not include these measurements? However, what that sum does not consider is the effect of equipotential bonding.
For example, I tested a property on a TT a couple of weeks ago. Ra (Ze by any other name) was 16 or 17 ohms, the live Zs tests were substantially lower than that. Using the formula that you have been told to use would have given you a higher result.
Moral of the story? Always measure.
Why do you think that the Zs measurement needs to be taken at the last point of utilisation? (GN3 dictates that it is measured at every point and the highest value recorded but we all know what happens in the real world)

 

[Des 56]

1. why do you need to test at the last point?That is where the highest reading will be obtained,
the highest reading is used to compare to maximum values

2. does this test take in to account the ze and r1/2? Zs is the reading obtained which is a value of Ze with R1+R2 adding to it and other parallel earth paths (like services) reducing this impedance

 

[trev]

By the way Jake, apply to join the trainee section. It'll help you a lot

 

[Plonker 3]

Ze+(R1+R2)=Zs is only used in designing circuits as you will always have parallel paths which will reduce the Zs value.

 

[GMES]

Guys remember his 1st 6 posts have to be approved before he can answer, I will try to stick around to speed it up.

 

[trev]

Ze+(R1+R2)=Zs is only used in designing circuits as you will always have parallel paths which will reduce the Zs value.

It seems that the pups are being taught that as the way to get Zs now though mate.

 

[GMES]

By the way Jake, apply to join the trainee section. It'll help you a lot

Trev I put him a link in my opening post.
How are you anyway fella.

 

[GMES]

Jake if you post 5 quick replies it should let you post normally after that.

 

[GMES]

It seems that the pups are being taught that as the way to get Zs now though mate.

Trev you are correct I had a lad for work experience a while back and he said they try to keep them away from carrying out a zs due to it being a live test.
Ridiculous if you ask me, it is the easiest and quickest test to preform.

 

[Plonker 3]

It seems that the pups are being taught that as the way to get Zs now though mate.

To a extent you can use it on site, as you obtain Ze anyway and depending on your preferred method of dead test (R1+R2) so you can use the values to ensure that your circuits comply when you carry out a actual Zs test

 

[trev]

Trev I put him a link in my opening post.
How are you anyway fella.

Not bad mate, just getting quietly bladdered

 

[trev]

If you haven't got the sense to carry out a live test properly and safely then you have no business poking around in anything electrical imho.

 

[Plonker 3]

Trev you are correct I had a lad for work experience a while back and he said they try to keep them away from carrying out a zs due to it being a live test.
Ridiculous if you ask me, it is the easiest and quickest test to preform.

God help them then when they remove a cover off a C50 DB for the first time lol.

 

[GMES]

Not bad mate, just getting quietly bladdered

That's not like you lol.

 

[GMES]

If you haven't got the sense to carry out a live test properly and safely then you have no business poking around in anything electrical imho.

God help them then when they remove a cover off a C50 DB for the first time lol.

I feel sorry for the lads learning though, because naturally they are going to follow the advice of their tutors, until they can get on site a be guided by an Electrician.

 

[trev]

That's not like you lol.

Just trying to get numb mate