Zdb Question

[Lee2610]

I have a question about testing on a consumer unit from a distribution board, how would you do the Ze test at the second consumer unit ? I have heard a term Zdb but cant really find information on this.

 

[Hellmooth]

Ze would be taken as normal, it would be named Zdb though.

 

[hightower]

I'm just a trainee who hasn't come across this yet but surely you just test at the second db as you would the first and that is your ze?

 

[HappyHippyDad]

As above. Ze is so named as it is 'external' earth fault loop impedance, so you can't give the same name to a earth fault loop impedance of another CU that is fed from the main CU as part of the earth fault would be 'internal' where it travels to and from the main CU from the sub board. The test would be carried out in the same way as when you conduct a Ze but the result would be called Zdb.

 

[7029 dave]

Ze would be measured at the origin and would require all parallel earths to be disconnected.
This submain, is it fed from a distribution board? If so then treat it as a circuit and measure earth fault loop impedance at the incoming terminals with all earthing and equipotential conductors in place. This measurement would be Zs or Zdb, both the same.

 

[davesparks]

Ze would be taken as normal, it would be named Zdb though.

No, Ze is taken at the incoming supply terminals with earthing conductor disconnected.
Zdb is the Zs at the incoming terminals of a DB with all bonds etc connected (Zs of the submain fed via one)

 

[Deleted member 26818]

The old test result forms used to ask for Ze at origin, the new ones ask for Zs at DB.
Which to my mind is an improvement.
Still put Ze on the first page.

 

[Andy78]

As a simplified definition, Zdb is the name given to the Zs of the distribution circuit supplying the db.

 

[LeeH]

The old test result forms used to ask for Ze at origin, the new ones ask for Zs at DB.
Which to my mind is an improvement.
Still put Ze on the first page.

I remember this confusing me as an apprentice.

 

[uxb443]

Zdb at the second consumer unit.
I assume you have the Ze reading for the main distribution board?
If you add Ze for the main distribution board to the R1 + R2 for the tails from Main Distribution board to the consumer unit that will give you Zdb.

Resistance aΩ + Resistance bΩ = Resistance cΩ.

 

[davesparks]

Zdb at the second consumer unit.
I assume you have the Ze reading for the main distribution board?
If you add Ze for the main distribution board to the R1 + R2 for the tails from Main Distribution board to the consumer unit that will give you Zdb.

Resistance aΩ + Resistance bΩ = Resistance cΩ.

No it won't, Zdb will also include the parallel paths to earth created by bonding extraneous parts.

 

[LeeH]

Zdb is Zs so it is acceptable to measure or calculate. That's what the BYB says isn't it?

Any bonding is at the MET or EMT, not any other boards.

Tin hat on obviously!

 

[uxb443]

No it won't, Zdb will also include the parallel paths to earth created by bonding extraneous parts.

No it won't, Zdb will also include the parallel paths to earth created by bonding extraneous parts.

I was asked this precise question by a C&G examiner this week. The examiner said my answer was correct.

Measuring EFLI at the secondary consumer unit leaving all parallel paths connected would give a lower resistance than the sum of Ze from the main board and adding the R1+R2 for the tails to the secondary consumer unit. The purpose of Ze and therefore Zdb at the secondary board is to give the worst case scenario should, for some odd reason, all the parallel paths disappear. The Zdb can then be used for objective calculations for the operation of RCDs (for example).

Yes, a lower reading would make downstream circuits more likely to pass, but surely that is not the aim of the exercise?

 

[HappyHippyDad]

I was asked this precise question by a C&G examiner this week. The examiner said my answer was correct.

Measuring EFLI at the secondary consumer unit leaving all parallel paths connected would give a lower resistance than the sum of Ze from the main board and adding the R1+R2 for the tails to the secondary consumer unit. The purpose of Ze and therefore Zdb at the secondary board is to give the worst case scenario should, for some odd reason, all the parallel paths disappear. The Zdb can then be used for objective calculations for the operation of RCDs (for example).

Yes, a lower reading would make downstream circuits more likely to pass, but surely that is not the aim of the exercise?

I asked my assessor about 'calculated' and 'measured' Zs as my reasoning was as above. The calculated method [Ze+(R1+R2)] is likely to be a higher resistance value than the measured value due to the bonding still being connected with a 'measured' value. He stated that a 'measured' figure should be given although did not explain why.

We remove the bonding cables to get an accurate Ze and because we cannot presume the resistance value of the bonding will be a stable figure, however when we measure Zs we leave the bonding in place so we are using part of that 'instabilty' in our 'measured' value of Zs??

On ocassion the MFT will not record a Zs, then I can't see another option to using a calculated figure.

 

[Simonslimline]

I asked my assessor about 'calculated' and 'measured' Zs as my reasoning was as above. The calculated method [Ze+(R1+R2)] is likely to be a higher resistance value than the measured value due to the bonding still being connected with a 'measured' value. He stated that a 'measured' figure should be given although did not explain why.

We remove the bonding cables to get an accurate Ze and because we cannot presume the resistance value of the bonding will be a stable figure, however when we measure Zs we leave the bonding in place so we are using part of that 'instabilty' in our 'measured' value of Zs??

On ocassion the MFT will not record a Zs, then I can't see another option to using a calculated figure.

A calculated Zs will not pick up any high resistances through protective devices etc.