Bs 7671 on transformer circuit breaker selection

A very specific and technical topic!

In the UK, the Institute of Engineering and Technology (IET) publishes the Wiring Regulations, commonly referred to as BS 7671. The standard provides guidelines for the safe installation of electrical systems in buildings.

When it comes to transformer circuit breaker selection, BS 7671 provides guidance on the selection of circuit breakers for transformer circuits. Here are some key points to consider:

1. Transformer rating: The circuit breaker should be rated for the maximum current that the transformer is designed to supply. This is typically specified by the transformer manufacturer.
2. Fault current: The circuit breaker should be able to withstand the maximum fault current that may occur in the circuit. This is typically calculated using the short-circuit current calculation method outlined in BS 7671.
3. Inrush current: The circuit breaker should be able to withstand the inrush current that occurs when the transformer is first energized. This is typically a high current that decays rapidly.
4. Overcurrent protection: The circuit breaker should provide overcurrent protection for the transformer and any connected equipment. This means that it should be able to trip quickly enough to prevent damage to the equipment in the event of an overcurrent.
5. Type of transformer: The type of transformer being used (e.g., dry-type, oil-immersed, or liquid-filled) can affect the selection of the circuit breaker. For example, oil-immersed transformers may require a circuit breaker with a higher fault current rating.

In terms of specific requirements, BS 7671 recommends the following:

• For transformers with a rating of up to 100 kVA, a circuit breaker with a rating of up to 63 A should be used.
• For transformers with a rating of 100 kVA to 500 kVA, a circuit breaker with a rating of up to 125 A should be used.
• For transformers with a rating above 500 kVA, a circuit breaker with a rating of up to 250 A should be used.

It's important to note that these are general guidelines, and the specific requirements for a particular installation may vary depending on the specific circumstances. It's always recommended to consult the relevant sections of BS 7671 and to seek advice from a qualified electrical engineer if you're unsure about the selection of a circuit breaker for a transformer circuit.