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 tabulated value of Zs in accordance with BS 7671?

3 gives the maximum Z_s value of 1.37 ohms at a nominal voltage (U₀) of 230 V. This compares with 1.44 ohms in the same table of the previous version of BS 7671.

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 maximum measured value of Zs for a 20 a radial circuit protected by a bs88 3 fuse?

Here we find that the maximum Z_s value for 20 A BS 88-3 fuses is 2.04 ohms.

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.

Calculating Maximum Zs Values for Circuit Breakers and Other Devices

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 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.

What is the ZS value?

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.

How do you calculate Zs value?

Therefore, where reliable measured values are available for the external earth loop impedance (Ze) and for the loop resistance of the line and protective conductors (R₁ + R₂) of the circuit, it is permissible to derive the loop impedance of a circuit by using the following formula: Zs = Ze + (R₁ + 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 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).

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.

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 is a high Ze reading?

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's the difference between TNS and TNCS?

The differences between the TNS and TNCS earthing system

The major difference between these two methods of earthing is that you have a separate earth core back to the substation in a TNS whereas in a TNCS the earth and neutral are the same core (CNE).

Is TNS and TNCS PME?

Yes it does. PME supplies are TN-C-S but not all TN-C-S supplies are PME. You could have a TN-C-S supply direct from an on site transformer without the multiple earthing.

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 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.

How is ze value calculated?

Ze =R1 + R2. While doing testing, if your not required to take insulation resistance or an rcd time.

Is it OK to calculate Zs?

It seems that nowadays it is acceptable across the board to calculate Zs. When I used to do a lot of testing, the contractor I worked for insisted on measured readings. That was the company line and therefore that is how I used to work. At that time a calculated Zs was seen as "lazy", so that is my mindset still.

What is maximum R1 R2?

Therefore the max expected R1 + R2 value would be 0.73 + 1.20 / 4 = 0.48 Ohms.