# What is the maximum ZS permitted by bs7671?

## What is the maximum ZS permitted by bs7671?

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

## What do I do if my 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 a BS88 fuse used for?

A BS88 fuse contains a sacrificial link that breaks during an overload or short circuit scenario to protect the rest of the electrical system. BS88 fuses are used in low voltage industrial and general-purpose applications and meet the requirements of BS88 and IEC60269.

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

The maximum external earth fault loop impedence Ze is 0.35 ohms. The maximum prospective fault current is 16 kA.

### How do you calculate actual Zs?

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).

### How do you calculate max Zs?

Methods for determining Zs

1. Direct measurement of Zs using an earth fault loop impedance test instrument.
2. Adding the result of the value of external earth fault loop impedance (Ze) to the measured value of (R1 + R2) at the end of the circuit.
3. Measurement of Ze.
4. Measurement of (R1 + R2)

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. This number should be higher than the highest measured Zs value for the given circuit.

What does R1 R2 prove?

For each outlet on a circuit, the resistance of both the live and earth conductors together (R1+R2) is measured back to the source. This test also helps identify polarity problems – for example, if a live wire has accidentally been switched with a neutral wire somewhere in a circuit.

#### What type of fuse is BS 88?

The fuse complies with standard EN 60269-2 section II and standard BS 88 part 2. This fuse range insures an excellent current limi- tation for all overloads on a large range of appli- cations. Their size cannot allow exchange by other fuses of higher rating in their range.

#### How do you calculate Zs?

What is the maximum Ze for a TT system?

200Ω
The IEE recommended figure for external loop impedance (Ze) for a T-T system is up to 200Ω although current standards define a higher maximum value. Due to Ohm’s law, this means low fault currents can be expected when you have a high resistance or impedance.