Basic help please

[newtothis]

After testing a lighting circuit
Got feedback saying my result is wrong / iffy :nopity:
B6 MCB, 1mm 2 cable
Measured RI+R2 = 0.69
Measured Ze 0.34
Measured Zs 0.88


Calculated Zs by adding Ze + (R1+R2) = 1.03
Allowed 5.82 80% rule

I can't work out why my result is wrong - what am I missing?

 

[Leesparkykent]

Your measured Zs will sometimes be lower than your calculated Zs due to parallel paths.

 

[newtothis]

Your measured Zs will sometimes lower than your calculated Zs due to parallel paths.

Thanks for replying. On my schedule of tests results none of the calculated Zs value are exactly the same as the measured. This is the only one flagged up. Can't work out why.

 

[Richard Burns]

What length of circuit is being tested, the result tend to indicate about 20 m so if the circuit is only 6 m then there seems to be additional resistance, if it is a test rig then the designed length may be low.

Other than that was this circuit one with the biggest difference between calculated and measured Zs, in general, as a guideline, one should expect the parallel path resistance change to be of a similar range across circuits, even though there are many factors that can affect this.

 

[newtothis]

Thanks Richard,
I've put the results on a spreadsheet to show the rest of the results the differ col shows..

 

[Richard Burns]

Just looking at the table and comparing to the bathroom lighting circuit, that has a similar measured Zs, so one would possibly expect a similar R1+R2.
Yours is only 0.18 Ω different, though that could be very significant compared to the measured values. I would expect a minor contact resistance problem.
Overall looking at those results I would not look twice (except for the 80% Zs for the water heater and the emergency lighting run in singles!:angel_smile and would think the results were OK, even good.

It is possible that the results do match up with other resistance measurements recorded for this circuit.

 

[newtothis]

Thanks for having a look Richard.
It was on a test rig, they put resistors in the circuits.
Hence the lengths having no bearing on reality.
I did this as a trial run just before doing my 2392 last Friday, passed that OK
I've paid and signed to do 2394 at my local college.

LOL well spotted on the water heater 80% Max Zs that's a typo

 

[Richard Burns]

As it is a test rig they would know the resistance that is recorded by many students and I would expect that yours was higher than the other readings given.
This would as I said before tend to indicate a contact resistance error, i.e the probes were not quite in good contact with the conductors. Always make sure you push firmly (but not hard enough to slip) on probes and compress/ twist/ wobble any clips or clamps firmly against the conductor before taking a reading.

As you have passed 2392 (well done:hurray you can now carry on to the real stuff of the 2394!!

Good luck with the 2394. (GN3, GN3, GN3)

 

[newtothis]

Always make sure you push firmly (but not hard enough to slip) on probes and compress/ twist/ wobble any clips or clamps firmly against the conductor before taking a reading.

I'm finding GS38 compliant test probes a bit restrictive, ha ha that's my excuse.
The three lead ZS at a ceiling rose seems, unnecessarily awkward to me.
I've been looking at getting the kewtech lightmate testing kit
Kewtech 5x Lightmate Kit - BC, ES, GU10, SES and SBC
LOL looking forward to getting into 3 phase

 

[Richard Burns]

GS38 probes are a nuisance, the hanging three probes from a ceiling is where the sprung hook probe becomes so useful

I was too cheap to buy the lightmates, just bought one lightmate and a lot of lighting adaptors!
However remember you still need to get to the earth somehow.

Three phase just means you need to do more tests!!

 

[newtothis]

ScrewFix / electroFix are doing the whole lightmate set for £47.49
so I thought I'd take a night off the beer and treat myself

Three phase just means you need to do more tests!!

Yes I hope to over-familiar the theory, and the diffs between 3 and single phase when I start the 2394

 

[davesparks]

Ouch, that's expensive! I got mine free when I bought the MFT from the wholesaler. Does nobody haggle properly anymore?

 

[newtothis]

Ouch, that's expensive! I got mine free when I bought the MFT from the wholesaler. Does nobody haggle properly anymore?

They're 55 sovs plus, everywhere else, you got a bargain there Davesparks.

I aint got a kewtech MTF. I got the impression they're not very fashionable. LOL !!

 

[davesparks]

They're 55 sovs plus, everywhere else, you got a bargain there Davesparks.

I aint got a kewtech MTF. I got the impression they're not very fashionable. LOL !!

Of course I got a bargain, that's the whole point of using local wholesalers!

I have got one, it's an improvement in the fluke 165x series rubbish but not as good as the megger gear

 

[newtothis]

I have TLC, N&E, CEF, Screwfix,and toolstation clustered within walking distance of my house.

[FONT=&quot]If everything’s the same price I go to SF purely on the basis; they have nice babes behind the counter.
Note to self: don’t post when drinking :tounge_smile:[/FONT]