ELECTRONIC TEACHING RESOURCES

Why does it matter?

The BS7671 regulations state the maximum values of Zs that are permitted in an installation in order to allow protective devices (fuses and breakers) to disconnect within a certain time period.

This information can be found in Appendix A (tables A1 to A4).

Examples

1. 1.What is the maximum permissible value of Zs in a circuit protected by a 16A type B MCB?

1. 2.What is the maximum permissible value of Zs in a circuit protected by a 25A type B MCB?

1. 3.What is the maximum permissible value of Zs in a circuit protected by a 16A type C MCB?

1. 4.What is the maximum permissible value of Zs in a circuit protected by a 16A type D MCB where a disconnection time of 0.4 seconds is required?

1. 5.What is the maximum permissible value of Zs in a circuit protected by a 5A BS88-3 fuse where the CPC is 1.5mm and requires a disconnection time of 0.4 seconds?

1. 6.What is the maximum permissible value of Zs in a circuit protected by a 5A BS88-3 fuse where the CPC is 1.5mm and requires a disconnection time of 5 seconds?

1. 7.What is the maximum permissible value of Zs in a circuit protected by 45A BS88-3 fuse where the CPC is 2.5mm and requires a disconnection time of 5 seconds?

Trickier ones!

1. 1.A ring-final circuit in a domestic installation has an R1 value of 0.34Ω, and R2 value of 0.56Ω. The installation is supplied via a TT connected supply with a Ze of 22Ω. Calculate the Zs and determine if the value meets regulations if the circuit is protected by a 32A type B MCB.

1. 2.A radial circuit in a domestic installation has an R1 value of 3Ω and a R2 value of 25Ω. The installation is supplied via a TN-C-S connected supply with a Ze of 1.2Ω. Calculate the Zs and determine if the value meets regulations if the circuit is protected by a 32A type B MCB.

1. 3.A radial circuit in a domestic installation has an R1 value of 1Ω and a R2 value of 1.7Ω. The installation is supplied via a TT connected supply with a Ze of 200Ω. Calculate the Zs and determine if the value meets regulations if the circuit is protected by a 5A type B MCB.