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  9. Overcurrent Protector

Is an Overcurrent Protector Needed for the United Kingdom?

Video Transcript

Ralph Bright—
Hello, I’m Ralph Bright, Vice President of Marketing and IT at Interpower.

We have real people answering the phones each day at Interpower which means we receive a number of product design questions. Usually if a question has been asked by one person, it means that others also have the same question.

One commonly asked question is: “Why would I still need an over current protector for my equipment that I am sending to the United Kingdom, since that plug already has a fuse in it?”

To answer this question, we have Bob Wersen, President of the Interpower Group of Companies. Welcome, Bob.

Bob Wersen—
Thank you, Ralph.

Why is an over current protector for equipment still needed when using the U-K plug?

Technically, the fuse in the U-K plug is designed to protect the cable between the plug and the equipment in the case of a sudden short circuit. The key objective is to prevent the cable from overheating and causing a fire. If the equipment is connected to a U-K Ring Main system, that sudden short circuit is sourced from two directions which can delay how soon the building miniature circuit breaker or fuse can clear the fault.

Can you give an example?

Over current protection in equipment is not just for sudden short circuits but also for situations where equipment for various reasons draws more current than it is designed to handle. One example is a power strip with a U-K fused plug. Such a power strip could have too many devices connected to it resulting in an over current situation which the plug fuse may allow to continue to occur for a given amount of time. U-K plugs use B-S 1362 fuses which are not highly sensitive to overload currents and in fact will operate for half an hour even at 1.6 times the rated current. Their primary purpose is to protect against short circuit currents.

Are there other countries, besides the United Kingdom, that use a fused plug?

The United Kingdom is the only country that requires a fused power plug. The British use a ring wiring system in their houses and buildings, which provides a secondary protection device at the plug to minimize safety hazards.

Circuits leave the local branch protection device, travel out to the loads (such as outlets or lamps) and then return to the circuit protection device. A fault condition at an outlet, for example will be sourced with current from both directions. This minimizes the amount of heat generated in the conductor, as the fault condition occurs, but before the circuit protection device can clear the fault. By minimizing the heat generated, the degradation of insulation (which accompanies overheating due to repeated fault conditions) is also diminished, improving the long-term safety of the insulation system.

Until the circuit protection device clears the fault, however, the fault condition is sourced from two different directions in the supply system. There is, therefore, a much greater potential fault current condition. The secondary protection device at the plug minimizes the safety hazard this condition creates, hence the power plug fuse.

Thank you Bob for the information and thank you for joining us today.

If you have a question that you would like answered, please let us know by e-mailing us at or calling us at 1–800–662–2290. For more information, check our website at