Measuring Zdb

[Spearmint]

Evening chaps, thought I'd post this here to help potentially successful trainees in the future as I'm 99% sure this isn't covered in the courses (certainly not in mine anyway).

Measuring Ze and Zs is straight forward, and we all have in our heads Zs = Ze + (R1+R2) BUT what about measuring these at sub mains? I've heard of taking the usual Ze and adding this to the R1+R2 supply to give you the "Zdb" for that particular DB then using that Zdb as the Ze for the next DB and adding its circuits R1+R2 giving you the Zs. However how can you take a direct reading of Zs from a sub main circuit? I gave it a whirl on site today and ended up with ~180Ω from a socket outlet which clearly isn't right, it's TN-C-S btw.

Setup goes: DB1 > RCBO > DB2 (which has it's own RCD covered circuits).

As far as I'm aware we're shown measuring Zs and Ze and their relevance to R1+R2 but only at a single DB.

Cheers in advance!

 

[Simonslimline]

When measuring ZDB you just do the test like you would a Zs test.

Is it a possibility that the part of the install you took your reading from is using an electrode for earthing?

 

[Spearmint]

When measuring ZDB you just do the test like you would a Zs test.

Is it a possibility that the part of the install you took your reading from is using an electrode for earthing?

No it's TN-C-S, not TT. Carrying out what I assume to be a direct measurement (MET connected on main DB which in turn connected to sub main, energised etc.), I end up with ~180Ω at each socket outlet. That value is way off the scales though, I must be missing something surely?!

 

[Simonslimline]

So are you saying you are testing sockets which are connected to DB2 , and the submain for DB2 comes from DB1?

What are the readings of Zdb2 , ie PFC and impedance ?

 

[Spearmint]

So are you saying you are testing sockets which are connected to DB2 , and the submain for DB2 comes from DB1?

What are the readings of Zdb2 , ie PFC and impedance ?

Yes, sockets from DB2 which is feed from DB1. Couldn't tell you the Zdb etc. from DB2 as I only had a few minutes and went for the "direct" route, will be back sometime next week hopefully so will report back. Is there something I'm missing or can the Zs from a sub main circuit only be calculated and not measured? Surely not...

 

[Simonslimline]

My thinking is to take readings from DB2 (PFC & Impedance) if you get a reading near your 180ohms then you know it is probably a problem with the submain and not the sockets.

Is the submain SWA and is the armouring being used as the earth?

I would be taking direct measurements and not calculating if it were me mate

 

[Paul.M]

There is a way of figuring out the Zdb. Lets just post up some numbers to help explain.

Ze = 0.10
Zs for a socket circuit fed from Db2 = 0.50
and the r1+r2 from the same circuit = 0.35

Therefore Zs - (r1+r2) - Ze = 0.05 for your Zdb2. Use this as an expected result before testing the Zdb, you should get a reading of 0.15

At this socket fed from Db2 the formula is Zs= (r1+r2) + Zdb.

Another formula that may help you whilst carrying out DB testing is this:- PFC = Volts divided by Ze....eg V= 230, Ze = 0.10 so your PFC should be 2.3 kA, try it next time your testing

 

[Simonslimline]

That was one of the handy things on the new 1700 series meggers, when doing Ze/Zs tests the PFC would show aswell.

 

[Paul.M]

My Metral does the same, also gives you the Isc when doing a Zs.

 

[Spearmint]

My thinking is to take readings from DB2 (PFC & Impedance) if you get a reading near your 180ohms then you know it is probably a problem with the submain and not the sockets.

Is the submain SWA and is the armouring being used as the earth?

I would be taking direct measurements and not calculating if it were me mate

Supply is coming via a 10mm² T+E with an additional 10mm² earthing cable back to DB1. I'd prefer to take a direct measurement over calculating too.

There is a way of figuring out the Zdb. Lets just post up some numbers to help explain.

Ze = 0.10
Zs for a socket circuit fed from Db2 = 0.50
and the r1+r2 from the same circuit = 0.35

Therefore Zs - (r1+r2) - Ze = 0.05 for your Zdb2. Use this as an expected result before testing the Zdb, you should get a reading of 0.15

At this socket fed from Db2 the formula is Zs= (r1+r2) + Zdb.

Another formula that may help you whilst carrying out DB testing is this:- PFC = Volts divided by Ze....eg V= 230, Ze = 0.10 so your PFC should be 2.3 kA, try it next time your testing

That makes sense, thanks. How would you go about taking a direct Zs measurement from an energised DB2 though rather than calculating? Without powering up, the meters just scream "no voltage" but when it's energised I ended up with the high reading. Or is calculating the only way?

 

[Paul.M]

Supply is coming via a 10mm² T+E with an additional 10mm² earthing cable back to DB1. I'd prefer to take a direct measurement over calculating too.

How would you go about taking a direct Zs measurement from an energised DB2 though rather than calculating?

You cant take a Zs reading from any DB! Think about, what a Zs reading is? It is the resistance of the Line (live aka R1) and CPC (earth aka R2) conductors which should have been tested already on your dead tests before energizing the circuit.

If the R1+R2 was lets say 0.30 Ohms and the Ze or Zdb was 0.15, you then have a Zs under live test of 0.45 Ohms for the furthest point of that circuit. Hope that makes sense? If not just ask

Without powering up, the meters just scream "no voltage" but when it's energised I ended up with the high reading. Or is calculating the only way?

Think your getting mixed up with Dead testing and Live testing. Does the above make sense to you all or not? I dont mind going through it with you all.

 

[Simonslimline]

Just remember that a Zs is taken with all protective bonding conductors connected, the reality is you will have parralel paths to earth so although the theory of Zs equals Ze + (R1 + R2), this may not always be the case .

 

[Spearmint]

Think your getting mixed up with Dead testing and Live testing. Does the above make sense to you all or not? I dont mind going through it with you all.

OK I think it's starting to sink in, I think what might be throwing me is the tester I used. It has one of those plug socket adapters which is what I was using on one of the RFC, so I was selecting the relevant function on the meter (trip lock) and plugging in. Both the DB1 and DB2 were energised upon testing and after a while it spits out 180Ω. R1+R2 came out low beforehand and didn't carry out a Ze so couldn't tell you that yet. I think I might be getting muddled up between the meter and the method maybe?

Other than that what you said makes perfect sense. If you were to take a Zs reading, what would you do?

Just remember that a Zs is taken with all protective bonding conductors connected, the reality is you will have parralel paths to earth so although the theory of Zs equals Ze + (R1 + R2), this may not always be the case .

They were all connected (on both boards), and the idea of parallel paths makes sense as other circuits / bonding would be excluded taking R1+R2.

 

[Simonslimline]

When taking a Zdb it is no different to taking it from say a lighting point or fused connection unit.

Set the tester to the appropriate loop setting (High or No trip) depending on the presence of RCD,s, Connect to the incoming line in the main switch and to the earth bar, hit test and wait.
Depending on the tester you have you may need to use 3 leads if an RCD is in circuit and connect the 3rd lead to the incoming neutral.

Hope this is useful

 

[Spearmint]

When taking a Zdb it is no different to taking it from say a lighting point or fused connection unit.

Set the tester to the appropriate loop setting (High or No trip) depending on the presence of RCD,s, Connect to the incoming line in the main switch and to the earth bar, hit test and wait.
Depending on the tester you have you may need to use 3 leads if an RCD is in circuit and connect the 3rd lead to the incoming neutral.

Hope this is useful

Got it! Penny has dropped with the Zdb, makes perfect sense now. I still don't get why I'd get such a high Zs reading testing all socket outlets on the new circuit though.

Sorry Paul I wasn't clear before. I wasn't trying to take readings FROM the sub main board, I was taking it from one of its socket outlets. Plug top adapter from meter into the RFC.

Following on from that won't I always get a high Zs reading seeing as the meter requires the board (and in turn boards) to be live whether using the "plug top" adapter or straight into the back of the sockets?

Apologies chaps if I'm seeming hard work, I'm just waiting for it to click in!