Stroma Certification provides some guidance on values of earth loop impedance (Zs) for legacy circuit breakers.
A frequent enquiry Stroma Certification receives is regarding the maximum values of earth loop impedance (Zs) for legacy circuit breakers manufactured to BS 3871. The maximum values of Zs for BS 3871 circuit breakers were published in The Wiring Regulations up until the 2001 edition of BS 7671.
The BS 3871 standard was replaced by the BSEN 60898 standard. Whilst these circuit breakers are obsolete, they are fully acceptable to remain in service and some may be more robustly constructed than their modern equivalents.
Automatic Disconnection of the Supply
To ensure Automatic Disconnection of the Supply (ADS) is achieved in the maximum permitted disconnection time, the maximum value of Zs can be determined from the formula Zs ≤ U0 x C min / Ia where Cmin = 0.95. This allows for fluctuations in supply voltage and Ia is the current that will cause the circuit breaker to operate in the required disconnection time.
When contractors wish to make additions to circuits protected by BS3871 circuit breakers, or are carrying out a periodic inspection and test of an existing installation, they will need to verify that the measured value of earth loop impedance will permit ADS.
Maximum values of Zs for generic BS 3871 circuit breakers
|In (A)||Type 1||Type 2||Type 3|
These values are valid for a disconnection time of up to 5s.
Fault current ratings
The prospective fault current ratings of BS 3871 circuit breakers are indicated by the letter ‘M’ followed by a digit. For example: M4.5 indicates a 4.5kA rating and M9 indicates a 9kA rating.
One of the more common types of BS 3871 circuit breaker found on installations is the Crabtree C50 which was produced in single phase and 3 phase versions. These are all Type 2 circuit breakers and their fault current rating can be determined by the infill paint in the operating handle. White paint infill indicates a 3kA rating and yellow paint indicates a 4.5kA rating.
The ‘Stablok’ circuit breakers, manufactured by Federal Electric Type, are indicated by the colour of the operating handle; white for Type 1, green for Type 2, orange for Type 3 and black for Type 4. These are 5kA rated.
A selection of BS 3871 circuit breakers.
Type 4 circuit breakers
BS 3871 circuit breakers were also manufactured as Type 4. Generic data cannot be used for Type 4 circuit breakers and the manufacturer’s data must be used to calculate maximum values of earth loop impedance. The more common forms of Type 4 are listed in the following tables, together with their maximum values of Zs, for 0.4s and 5s disconnection times.
The values of earth loop impedance shown in these tables must compensate for conductor temperature rise, if the measurement of loop impedance is taken at ambient temperature. A useful rule of thumb is to allow for a temperature rise from 20 degrees to 70 degrees by multiplying the listed value by 0.8. The measured value can then be compared to the compensated value.