Couple of testing questions ?

[Mikegh]

Apologies gone a bit rusty on this stuff:

1) (Max) Resistance of protective conductor:


An Installation with a sub-board all pvc wiring

Is the max protective conductor resistance = the resistance of the protective conductor on the submain + the highest resistance CPC on the sub-board

Above in steel with parallel paths, structural steel etc.
How is the max protective conductor resistance measured here ?
I know you can work out the earth conductor resistance using L-N loop tests but what figure is recorded


2) Can someone explain simply the L-E loop test for ring circuits
This is used to measure R2 ?

Cheers

 

[timhoward]

I admit I'm a bit confused.
1) - What are you trying to fill in, where? Are you trying to arrive at a value for the R2 column for a circuit fed from the submain consumer unit?
And are your two questions:
a) should you add the submain's own CPC resistance
b) how do you measure if the CPC is utilising steel conduit/containment?

2) Are you meaning the figure of 8 test (dead test), which is used to measure R1+R2? Or a loop test (live test) to obtain the measured Zs?

 

[pc1966]

2) Can someone explain simply the L-E loop test for ring circuits
This is used to measure R2 ?

If you are checking just at the CU you would measure the loop resistances of the ring:

• r1 = live to live
• r2 = cpc to cpc
• rn = neutral to neutral

Typically you should see that r1 and rn are almost the same, and r2 is in the ratio for the type of cabling used. For example, in the UK 2.5mm T&E has a 1.5mm CPC so the resistance ratio should be around 2.5/1.5 = 1.67

For example, if you measured r1 = rn = 0.6 ohm then you would expect r2 to be 0.6 * 1.67 = 1 ohm. Of course it might be lower due to parallel earth paths, and the odd 0.05 difference is not unreasonable when comparing due to meter accuracy, joints, etc.

In terms of the equivalent R1+R2 that is (r1+r2)/4 since the worst-case location for fault impedance on the RFC is the furtherest point, and at that point each 'leg' has (r1+r2)/2 due to being half way round the ring, and as they are in parallel there it is further halved leading to (r1+r2)/4.

 

[timhoward]

What @pc1966 said, in video form:

 

[pc1966]

The "figure of 8" test mentioned by @timhoward is more work, but equally more thorough, and typically used for initial verification.

Here you link the ring ends at the CU with L1 - CPC2 and L2 - CPC1

Then you measure the L-E resistance at each socket outlet, and each measured value is the R1+R2 value and they should all be identical (to measurement accuracy) and at (r1+r2)/4.

But what this test also verifies is socket polarity (very important for safety as only fused in the line path) and if there are any extra resistances, for example due to a spur or just a dodgy socket. Because of the test cross-linked ring structure each socket should measure the same, so it is easy to spot even a small extra resistance that should not be there.

So while it is more time consuming to do, it also provides exceptionally good fault coverage (more so if you can be bothered to do the same for L&N to verify neutral path resistance).

 

[Deleted account]

Are we talking earthing conductor, or bonding conductors?

 

[Mikegh]

I admit I'm a bit confused.
1) - What are you trying to fill in, where? Are you trying to arrive at a value for the R2 column for a circuit fed from the submain consumer unit?
And are your two questions:
a) should you add the submain's own CPC resistance
b) how do you measure if the CPC is utilising steel conduit/containment?

2) Are you meaning the figure of 8 test (dead test), which is used to measure R1+R2? Or a loop test (live test) to obtain the measured Zs?

Max resistance of protective conductor in the installation

Maybe they don't have that on the UK sheets, anyway it's yes to A and B

 

[Mikegh]

The "figure of 8" test mentioned by @timhoward is more work, but equally more thorough, and typically used for initial verification.

Here you link the ring ends at the CU with L1 - CPC2 and L2 - CPC1

Then you measure the L-E resistance at each socket outlet, and each measured value is the R1+R2 value and they should all be identical (to measurement accuracy) and at (r1+r2)/4.

But what this test also verifies is socket polarity (very important for safety as only fused in the line path) and if there are any extra resistances, for example due to a spur or just a dodgy socket. Because of the test cross-linked ring structure each socket should measure the same, so it is easy to spot even a small extra resistance that should not be there.

So while it is more time consuming to do, it also provides exceptionally good fault coverage (more so if you can be bothered to do the same for L&N to verify neutral path resistance).

Yes those are the tests and im familiar with the L-N setup
So as in the L-N the readings are the same at each outlet for L-E

I don't quite understand this (r1+r2)/4

What is the formula for r2 using this method ?


Edit posted this late , I'll read back over the last few posts
Cheers

 

[Mikegh]

So how is the r2 measured on a board with parralel paths on the final circuits ?

Cheers

 

[timhoward]

( Electrical Safety and Testing Standards - RECI Presentation - https://analytical-testing.ie/blog/electrical-safety-testing-standards/ )

I've had a read through your tests, and they are basically the same idea as what we have to do.
All I can say is that if it were a UK sheet, the sub main CU get's it's own sheet, and you would record the earth loop impedance at the incomer of the sub main DB at the top of the sheet.
Then for each circuit you would record either just the resistance of the circuit CPC (R2) on the circuit itself, or the combined resistance of the Live and CPC (R1+R2).
Then we also note the Zs which is the total, either measured like your test 8 "Line/Earth fault loop impedance" or calculated by adding the incomer impedance to the R1+R2.
I've never seen one of your test sheets though so hopefully someone local can comment!

So how is the r2 measured on a board with parralel paths on the final circuits ?

If the CPC is not a wire, then the R2 can be measured with a long wander lead, or a temporary link made at the circuit end and R1+R2 measured at the consumer unit. Just the same as if it were a wire really.
Disconnecting bonding and incoming earth at the sub-main (obviously with it isolated) can remove the parallel paths from the measurements.