Do You.. CALCULATE or TAKE ACTUAL READINGS ??

[Risteard]

TBH, I dont know why you can do it the one way, but not the other. Refering to my previous post, unless you are going to disconnect ALL parallel paths when doing your R1+R2, then there is no difference. I would argue that a loop test MUST be carried out, as this provides a real test, actually putting a current on the CPC.

They both put a current on the cpc. However for one test it's only in the region of 100-200mA at 4-24Vd.c.

 

[Jason Smith]

My inspector stated he wanted real readings on certs, (niceic) not calculated ones or ones supplied by the electric peeps. Calculations only to prevent you having to work live. EWR regs and all that .

 

[WayneL]

Do you all agree the same then ?

That.. ZS - ZE does NOT = R1+R2 ????

I know it may not be as accurate as taking a reading but its still a valid calculation isnt it ???

I cant belive im even questioning all collage tutors/ lecturers and assessors..

Hi again,

The reason is R1 is the 'resistance' of the line conductor and R2 is the 'resistance' of the Cpc - Zs is the 'impedance' of the circuit - in other words it includes all your parallels..... SWA armouring, steel conduit, supplementary bonding etc.

There is no point doing the calculation Zs - Ze because you don't need the answer.
You have to remember what you are doing by measuring R1 + R2 .... continuity of Cpc and polarity, doing the reverse calculation doesn't give you either of these and so is pointless.

The only time an accurate measurement of R1 + R2 can be taken is during installation (or calculated from circuit length) - any other time it's not worth doing. The only benefit R1 + R2 gives you in a periodic is that you can confirm polarity.

Your better off measuring R2 (to confirm continuity)and then measuring Zs

 

[Guest123]

TBH, I dont know why you can do it the one way, but not the other. Refering to my previous post, unless you are going to disconnect ALL parallel paths when doing your R1+R2, then there is no difference.

I would disagree, IMO no parallell paths exist when measuring the R1+R2 of a circuit as the line and cpc are disconnected at one end at least, maybe both depending on the circuit, eliminating any external parallel paths, theres only one way that measurement will travel. Metal faceplates, fittings, and the like dont come into it as there is no path between line and cpc for the reading to follow and no path to elsewhere either.


As I mentioned before......unless you measure Zs with all parallell paths removed, transposing the formula will not give an accurate reflection of the R1+R2 for a circuit.

 

[Davey101]

Not sure I follow. Consider a radial circuit wired in T+E, feeding 5 FCU's, connected to indoor air condensors, pumps, whatever. Run a cpc to the first FCU, but disconnect the outgoing cpc. Connect all FCU's to the (metal) equipment. Would you expect to get an R1+R2 at the last FCU?
How about a lighting circuit, wired in pvc conduit, feeding floru's mounted on perlins. Disconnect all cpc's, except the one going to the first fitting. Now do an R1+R2 on the last fitting. Parrallel earths are everywhere, although less so in domestics. Add in metal trunking/tube,water pipes, steel, ceiling grids etc etc and it gets worse/better.

The only way to eliminate parrallel earths is to sepearte cpc's from eqipment, or only have one accessory per cicuit, and dissconnect at both ends as you say.

BTW, I'm not suggesting that Zs-Ze is the correct way to go, the most accurate, or anything else, just that without disconnecting cpc's, you are not really doing a true R1+R2.

 

[Guest123]

I appreciate what you're saying mate, I really do but.....the test is between line and cpc of any one circuit that are linked together at one end and you are measureing from the other, the test couldn't take in those parallell paths as they are not connected to the line of the circuit under test only the cpc.

It's hard to explain but the circuit you are creating is only made up of the line and cpc of the particular cable which are joined at one end usually the DB to make a loop, the other paths you mention are not linked to the line of that circuit.......do you see what I mean????

 

[stef]

I personally prefer to measure. With the R1 + R2 test you already confirm polarity and continuity of the CPC and the continuity of the ring. With calculation it is hard to prove, esp when/if it is inconvenient to do a Ze test (for example in a factory whewre you can't just go in and disconnect earth from a large DB). With measurement you are on the safe side (if you confident with your meter) on the other hand it is easy to type in a wrong number in a calculator. When I did my 2391 the trainer told us that calculation confirms the measurement. I agree with that.

 

[Davey101]

If you connect the line to the CPC at the fuseboard, then if the cpc is connected to any metalwork , which it usually will be, then the line is also connected to that metalwork.
Screw a metalclad lightswitch on to a piece of steel, i.e a steel pillar. Run a length of 1.5mm 6491B from the DB, to that lightswitch.
20 metres away, screw another metalclad lightsitch onto another steel pillar. Connect 6 inches of 1.5mm 6491B to the back of the switchbox.
Now join live and cpc together at the DB. Go to the 2nd lighswitch and connect your meter between live and the 6 inch piece of cable. What reading would you get?
Now run a length of 1.5mm 6491B between the 2 lightswitches, what reading would you get?
You could have 20 switches, with a cpc between each of them, everyone bonded to earth. Disconnect the cpc from any lightswitch, and the last cpc will still have continuity to earth, via the steel. Only if you disconnect the first or last cpc will you fail to get continuity (R1+R2).

I think we are perhaps thinking about different things when we say parallel paths.

 

[Guest123]

If you connect the line to the CPC at the fuseboard, then if the cpc is connected to any metalwork , which it usually will be, then the line is also connected to that metalwork.
Screw a metalclad lightswitch on to a piece of steel, i.e a steel pillar. Run a length of 1.5mm 6491B from the DB, to that lightswitch.
20 metres away, screw another metalclad lightsitch onto another steel pillar. Connect 6 inches of 1.5mm 6491B to the back of the switchbox.
Now join live and cpc together at the DB. Go to the 2nd lighswitch and connect your meter between live and the 6 inch piece of cable. What reading would you get?
Now run a length of 1.5mm 6491B between the 2 lightswitches, what reading would you get?

I think we are perhaps talking about different things when we say parallel paths.

I see what your syaing, particularly where steel conduit is involved (cant really be sure unless you disconnect what you are reading through), but in your example I would go to the switches on that theoretical circuit, unscrew the switch fronts, disconnect the earths from the backboxes to eliminate the paths and then test.

I was thinking more domestically when answering previously, something about a standard 3 bed house from earlier in the thread stuck in my head.

Assumptions are sometimes the mother of all cock up's.

 

[Davey101]

I see what your syaing, particularly where steel conduit is involved (cant really be sure unless you disconnect what you are reading through), but in your example I would go to the switches on that theoretical circuit, unscrew the switch fronts, disconnect the earths from the backboxes to eliminate the paths and then test.

I was thinking more domestically when answering previously, something about a standard 3 bed house from earlier in the thread stuck in my head.

Assumptions are sometimes the mother of all cock up's.

Absolutley mate, just wanted to explain myself, unless everyone thought(rightly or wrongly) that I was mad

Now, I'll put my hands up, and say that I do not always disconnect flexs, cpc's, or remove switches etc when testing R1+R2. What does the OSG, or regs say about this?

 

[Guest123]

Off top of my head OSG mentions something about method 1 for R1+R2 only being used for measurement of an all insulated installation, cant remember anymore.......grey matter aint what it used to be.