Measure or calculate your Zs?

spud1 · Feb 6, 2019

Hi again!

Which value is best to record in a certificate or report, a max measured or a calculated Zs?

I know Sparkies who do it both ways, but personally I take measured values at all the points of utilization to find the highest, then I calculate Ze+(my highest R1+R2) for the circuit and then I compare the max measured and my calculated values and record the highest of the two on the certificate.

Any thoughts?

happysteve · Feb 6, 2019

If safe to measure Zs and I've measured it, I put the highest measured value down. If not safe to measure, I calculate it from Zdb + (R1+R2).

Edit: if it's a TT system and (for example) Ra = 100Ω and the board is at the origin (so Ze = 100Ω ish) but Zdb goes down to, say, 20Ω once bonding is connected, I would use Zs = 20 + (R1+R2).

(tin hat on)

spud1 · Feb 6, 2019

happysteve said:
If safe to measure Zs and I've measured it, I put the highest measured value down. If not safe to measure, I calculate it from Zdb + (R1+R2).

Edit: if it's a TT system and (for example) Ra = 100Ω and the board is at the origin (so Ze = 100Ω ish) but Zdb goes down to, say, 20Ω once bonding is connected, I would use Zs = 20 + (R1+R2).

(tin hat on)

But would a calculated value not be a truer reflection of the circuits prospective max Zs i.e. with no parallel paths at play

happysteve · Feb 6, 2019

Yes, hence the tin hat.

In the example of a TT system, though - unless you're using a 300mA or 500mA RCD for fault protection, it's not going to make much difference.

Bonding should be in place for ADS. When you measure Zs, bonding is in place.

If you measure Zs for some circuits (sockets, say), with values all around the 20-odd ohms mark, and you decide to use "proper" Ze + (R1+R2) for those you calculate Zs for, they'd be around the 100-odd ohms mark, and it would look... odd. You'd not really be comparing like for like (between circuits).

spud1 · Feb 6, 2019

happysteve said:
Yes, hence the tin hat.

In the example of a TT system, though - unless you're using a 300mA or 500mA RCD for fault protection, it's not going to make much difference.

Bonding should be in place for ADS. When you measure Zs, bonding is in place.

If you measure Zs for some circuits (sockets, say), with values all around the 20-odd ohms mark, and you decide to use "proper" Ze + (R1+R2) for those you calculate Zs for, they'd be around the 100-odd ohms mark, and it would look... odd. You'd not really be comparing like for like (between circuits).

In your TT example the highest reading would always be the calculated value so that would be method I would use for all circuits, so I would be comparing like for like wouldn't I?

Also with regards to your comment on bonding, what would happen if the utility pipe were upgraded to plastic and the parallel path previously giving you 20 ohms disappeared?

Oops I havent got a tin hat!

happysteve · Feb 6, 2019

Yes, these are points I had thought of, and you're right (I almost added a "PS" along those lines but assumed you would respond as you have).

If, ultimately, the presence or absence of bonding would make a difference between requirements for ADS being met or not (let's say fault protection for a particular circuit is via a 500mA RCD only, and Ra>100Ω, but with bonding Zdb is <<100Ω) then this would require a different response, depending on whether it's a new installation or an inspection and test.

happysteve · Feb 6, 2019

At the end of the day, make a note on the test sheet to say what you've done, then it's clear for whoever reads it in the future.

spud1 · Feb 6, 2019

happysteve said:
At the end of the day, make a note on the test sheet to say what you've done, then it's clear for whoever reads it in the future.

Spot on cheers Steve

Deleted member 26818 · Feb 6, 2019

spud1 said:
Hi again!

Which value is best to record in a certificate or report, a max measured or a calculated Zs?

I know Sparkies who do it both ways, but personally I take measured values at all the points of utilization to find the highest, then I calculate Ze+(my highest R1+R2) for the circuit and then I compare the max measured and my calculated values and record the highest of the two on the certificate.

Any thoughts?

Measured.
It’s a schedule of test results, not a schedule of calculations.

davesparks · Feb 6, 2019

Record the measured value. One of the reasons we record the values is so that subsequent EICRs and other testing can be compared to the original test results to track any deterioration in the circuits.

spud1 · Feb 6, 2019

spinlondon said:
Measured.
It’s a schedule of test results, not a schedule of calculations.

But it is a combination of two measured values, which discounts potentially unreliable parallel paths giving a falsely optimistic reading?

spud1 · Feb 6, 2019

davesparks said:
Record the measured value. One of the reasons we record the values is so that subsequent EICRs and other testing can be compared to the original test results to track any deterioration in the circuits.

But wouldn't any deterioration in the circuit be identifiable in the compared R1+R2 measured readings?

Ian1981 · Feb 6, 2019

[ElectriciansForums.net] Measure or calculate your Zs?

For initial verification The regulations say that where the effectiveness of the protective measure has been confirmed at a point located downstream of an rcd the protection of the installation downstream from this point may be proved by confirmation of the continuity of the protective conductors.
An R1+R2 test confirms this.
Where the rcd is tested and satisfies fault or additional protection then further testing is not nessicary according to the above NICEIC tech article.
If there’s no rcd and the circuit relys upon an ocpd for fault protection then the zs shall be confirmed by the appropriate measures.
643.7.1

Take it how you will I’d still just calculate or measure the Zs personally and write it in the appropriate column on the schedule of test results
A controversial point maybe?

Deleted member 26818 · Feb 6, 2019

See, I don’t understand this urge to fill in all the boxes?
The whole point of the schedule is to record the values obtained from tests, that you’ve conducted (hence the name).
You don’t get a gold star or a Brucie Bonus for filling in all the boxes. Unless of course you calculate the R2.

The other thing is, on some circuits the measured value for Zs will be considerably lower than the calculated value due to parallel paths.
For instance, boiler circuits or immersion heater circuits, all those copper pipes and bonding conductors will lower the Zs.

ferg · Feb 6, 2019

My recollection from the 2391 is that zs should be measured when safe to do so and calculated otherwise.

The interpretation was that sockets would be measured using an adaptor. Pretty much everything else calculated.