How do you test Zs on a distribution board?

Zs is your total impedence of the circuit including your sub main. There is only one Ze which is at the origin of the supply. 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.

How do you test for 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 is the source to check values of Zs?

For overcurrent devices not covered by those tables, another reliable source of information on the limiting values of Zs must be consulted, such as the manufacturer's data. Where the protective device is a nondelayed RCD, the maximum value of Zs can be found from Table 41.5 of BS 7671.

Where is a Ze test carried out?

The first is Ze, the external fault loop impedance, which is typically measured at the distribution board or consumer unit where the supply enters the building.

How do you find maximum Zs?

Max Zs Calculation Formula

So for a Type B 32 Amp MCB to BS EN 60898 the calculation would be: ​5×32(In) = 160 Amps (la) 218.5(Uo) / 160 (la) = 1.36525 (rounded to 1.37) Maximum Zs Value.

Earth Fault Loop Impedance Test

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!

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.

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 does a successful 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.

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.

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).

What is difference between Ze and 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.

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 does Mccb calculate max Zs?

Max Zs for 60974-2 MCCB

Please ensure the tables are to the current version of the wiring regulations and factor in the Cmin factor @ 230V. The Cmin factor is 230 x 0.95 = 218.5 volts, therefore to calculate Zs: 230 x 0.95 = 218.5, 218.5 / la = Zs.

Is a ZS test a live test?

The presenter does live Zs testing at sockets with a plug top lead switching on the socket switch before taking the test. So this in the words of the Regulation would be "reasonable".

What is the maximum Zs for a TNCS system?

Protective Multiple Earthing (PME) (TN-C-S system)

The maximum external earth fault loop impedence Ze is 0.35 ohms.

What does ZS mean in electrical terms?

Zs represent earth fault loop impedance and consists of Ze and (R1+R2). R1 is the phase and R2 is the CPC. These are measurements of the fault current from the supply transformer to the building. It covers the entire electrical cycle.

How do you get Ze?

`Ze` can be obtained by enquiry to the electricity supplier or by direct measurement at the supply. On the C&G 2391 practical assessment you will be required to obtain the value of `Ze` by direct measurement.

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.