Question 1

What is an SPD?

Accepted Answer

An SPD (Surge Protection Device) is a device fitted in or near the consumer unit to protect the electrical installation and connected equipment from transient overvoltages - sudden, brief spikes in voltage that can be many times higher than the normal 230V supply. These spikes are caused by lightning striking near the supply network, by high-energy switching operations on the distribution network, or (less commonly) by direct lightning strikes. The SPD contains components (typically metal oxide varistors, MOVs) that conduct heavily at high voltages, diverting the surge energy harmlessly to earth before it reaches the consumer unit and the circuits beyond. Without an SPD, a large transient overvoltage can destroy sensitive electronics, damage appliances, and in severe cases cause fire.

Question 2

What types of SPD are there?

Accepted Answer

SPDs are classified into three types based on the level of surge energy they handle: Type 1 SPDs are the most powerful, designed to handle the energy from a partial lightning current (tested with a 10/350 microsecond impulse current). They are installed at the service entrance (the main incoming supply) and are required where the building has a lightning protection system. Type 2 SPDs handle the residual surge energy that passes through a Type 1 SPD or arrives via the distribution network (tested with an 8/20 microsecond impulse current). These are the most common type in domestic consumer units - they are installed at the distribution board (consumer unit) and protect the installation from surges arriving from the supply network. Type 3 SPDs are lower-energy supplementary devices fitted at the point of use (such as socket outlet SPDs or equipment inlet SPDs) to protect specific sensitive equipment. For most domestic dwellings, a Type 2 SPD at the consumer unit provides sufficient protection.

Question 3

When does BS 7671 require an SPD?

Accepted Answer

The 18th Edition of BS 7671 (2018) added regulation 443.4 which requires surge protection to be provided where the consequences of overvoltage are likely to result in serious injury, loss of human life, failure of public services, disruption to commercial or industrial activities, or loss of irreplaceable cultural heritage - or where the cost of equipment damaged would be disproportionate compared to the cost of protection. For domestic dwellings, BS 7671 requires the electrician to carry out a risk assessment under regulation 443.5. In practice, many electricians and consumer unit manufacturers now include SPDs as standard in new consumer unit installations, as the cost is modest and modern homes contain large quantities of sensitive electronics (computers, smart TVs, broadband routers, smart home systems) that are vulnerable to transient overvoltages.

Question 4

Can I add an SPD to an existing consumer unit?

Accepted Answer

Yes - Type 2 SPDs can be retrofitted to existing consumer units if there is a spare way (circuit position) available. The SPD connects between the live busbars and the neutral/earth, and typically requires one or two MCB positions to be dedicated to the SPD backup fuse or MCB. Some SPD modules are designed to fit in a DIN rail consumer unit without consuming an MCB position (they connect directly across the busbar). Type 2 SPDs have a visual status indicator (green window) that shows when the device has operated and the MOV element has been exhausted - the SPD must be replaced when this indicator shows. An electrician should assess whether the existing consumer unit earthing arrangement is adequate for SPD installation - a good, low-impedance earth path is essential for the SPD to divert surges effectively.

SPD

Just so you know...