What happens if Zs is too high?

If a Zs value is high, then you need to establish whether it is due to circuit conditions, supply conditions, or a fault. You can then descide the appropriate course of action. Just installing an RCD or RCBO may not be the solution in a deteriorating installation. It would appear that PME is psychic.

What is the maximum ZS permitted by BS 7671?

​The 1.37 ohms value is the 100% max Zs value which should be recorded on the test certificate.

What is the maximum permitted Zs value for a TNCS circuit?

The value of external earth loop impedance (Ze) measured or otherwise determined in accordance with Regulation 313.1 may differ from the applicable typical maximum value declared by the electricity distributor, which is usually: 0.8 Ω for TN-S system. 0.35 Ω for a TN-C-S system.

What is the maximum Zs for a lighting circuit?

The maximum Zs is 3.84 ohms, 2.88 after 3/4 rule.

What is the 80% ZS rule?

The 80% rule of thumb is only intended to be applied to a test result taken from an impedance test. You have to ensure that the tested value does not exceed 80% of the tabulated value.

Zs TOO HIGH What can I do?

What is a good Zs reading?

The highest measured Zs value for each circuit should not exceed 0.8 of the relevant value in the BS 7671 tables.

What does a high Ze reading mean?

High ZE readings essentially refer to earthing arrangements in the property. High readings could be due to a wiring fault on the earth cable.

What does a ZS test prove?

A Zs test is not to prove an earth connection exists, although it would do so, but to prove that the CPD would operate within the time required by BS7671. Smpling should be of circuits, not equipment of a circuit that is being tested.

Is Zs at DB the same as Ze?

The board fed by the sub main will have a Zs value at the board which will roughly equal Ze = R1 +R2. You measure the Zs at the incomming terminals of the sub board. This is sometimes also called Zdb. At the end of the final circuits fed from this board will be a Zs for each circuit.

How do you measure Zs?

Determining the measured value of Zs

– (R1 + R2) is the measured value of resistance (Ω) for the line conductor and protective conductor, at the most distant point or accessory from the distribution board or consumer unit.

What should my Ze be?

We all know the 17th edition states that the maximum ZE should be 0.35 ohms. That figure is for TNCS, you said your supply is TNS which should be a max. of 0.80 ohms.

What is electrical Zs?

Zs is the total impedance of the complete circuit and it is made up of: The impedance of the supply cables and transformer windings, Ze. The resistance of our circuit line conductor, R1. The resistance of our circuit cpc, R2.

How can earth fault loop impedance be improved?

Increase the size of the PE or PEN conductors and/or the phase conductors, to reduce the loop impedance.

Why would ZS be high?

If a Zs value is high, then you need to establish whether it is due to circuit conditions, supply conditions, or a fault. You can then descide the appropriate course of action. Just installing an RCD or RCBO may not be the solution in a deteriorating installation. It would appear that PME is psychic.

What is the maximum Ze for a TNS system?

Except in city centres the conditions assumed for a TN-C-S system are: The maximum external earth fault loop impedence Ze is 0.35 ohms.

What happens if house not earthed?

If the house is not earthed, people could get electrocuted. Without an earth connection, the safety switches will not work and an electrical fault could cause a house or appliances to become 'live' as the current flows to earth.

Should R1 R2 be lower than R1?

R1+R2 must not be higher than R1(ring circuit)on a PIR report...

What is a good earth loop impedance?

It is generally accepted that, where the measured earth fault loop impedance of a circuit is not greater than 80% of the relevant limit specified in BS 7671, the impedance can be expected to be sufficiently low under earth fault conditions to meet the relevant limit specified in BS 7671, and for the protective device ...

How do you measure ZS on a ring circuit?

To find the zs of a ring circuit you need to cross connect at the origin of the ring, ie, connect the incoming phase to the outgoing cpc and vice-versa, then measure between phase & cpc at all of the sockets, the one with the highest reading gives the r1+r2 figure, just add this to your ze figure and you`re there!

How do you calculate ZS from R1 R2?

To measure Zs do just that go to the highest point of R1+R2 on that circuit and carry out a live test. Record your result. 2. Calculate Zs by Zs=Ze+(R1+R2).

When carrying out a ZS test on a ring circuit which reading do you record on the test certificate?

Step 3: Press the TEST button. The measurement should be a low reading ohm value. Do not forget to record this value of `Ze` on the Electrical Installation Certificate.