Surge Pro­tec­tion

16th November 2020

Surge protection devices (SPD’s) are devices which have been designed to protect vulnerable equipment within your home or business from transient over-voltages (electrical surges). With ever new technical advancements, both our homes and businesses are becoming more and more advanced. Many products contained in both the home and business contain very complex circuitry, which can be vulnerable to a surge. In a home, a surge may cause unrepairable damage to expensive appliances. However, many businesses may feel a much larger effect from a surge. For example, if a factory is hit with a surge and doesn’t have adequate protection, then the whole factory process maybe stopped costing a business a significant amount of money. If a hospital had a surge, then the very equipment designed to keep people alive may stop working and people can even lose their lives due to damaged equipment.

What are transient voltages?

A tran­si­ent over­voltage or electrical surge is a short dur­a­tion in­crease in voltage meas­ured between two or more con­duct­ors. This increase in voltage can measure from mi­cro­seconds (mil­lionths of a second) to a few mil­li­seconds (thou­sandths of a second) in dur­a­tion. This, however, is long enough to damage or destroy vulnerable equipment.

Please note, surge protection devices do not protect your home or business from a direct lightning strike. If such protection is needed, then you may need to consider lighting protection. SPD’s only protect your equipment from damage due to electrical surges, which can however often be caused by lightning strikes. There are also other situations which can cause transient voltages, such as the switching of transformers and motors.

18th Edi­tion Re­quire­ments

Current 18th edi­tion BS 7671 now states

Pro­tec­tion against tran­si­ent over-­voltages shall be provided where the con­sequences caused by an over-­voltage could

(i) res­ult in ser­i­ous in­jury to, or loss of hu­man life, or

(ii) res­ult in in­ter­rup­tion of pub­lic ser­vices and/​or dam­age to cul­tural her­it­age, or

(iii) res­ult in in­ter­rup­tion of com­mer­cial or in­dus­trial activ­ity, or

(iv) af­fect a large num­ber of co-loc­ated in­di­vidu­als.

For all other cases, a risk as­sess­ment shall be carried out by a competent person ac­cord­ing to Reg­u­la­tion 443.5. The risk assessment shall be per­formed in or­der to de­term­ine if pro­tec­tion against tran­si­ent over-­voltages is re­quired. If the risk as­sess­ment is not per­formed, the elec­trical in­stall­a­tion shall be provided with pro­tec­tion against tran­si­ent over-voltages.

For a single dwell­ing unit it will be a de­cision for the house owner to make whether they con­sider the small ad­di­tional cost of the surge pro­tec­tion device jus­ti­fied to pro­tect their in­stall­a­tion and equip­ment against these dam­aging over­-voltages.

Surge Protection Device

Types of SPD’s 

There are 3 common types of SPD’s and each has its use or place to fit within an electrical installation. Depending on the electrical installation, it may not be necessary to install all types of SPD. For example, it is not particular common to install type 1 SPD’s within a domestic household. However, you will need to consult a qualified electrician who will make this decision.

Type 1

An SPD which can dis­charge par­tial light­ning and discharge the back-current from lightning spreading from the earth conductor to the network conductors. This SPD, if re­quired, will be in­stalled in the primary dis­tri­bu­tion board, at the ori­gin of the elec­trical in­stall­a­tion. A Type 1 SPD does not in it­self of­fer the re­quired pro­tec­tion level and must be used in con­junc­tion with co­ordin­ated type 2 devices. An in­stall­a­tion with a light­ning pro­tec­tion sys­tem will re­quire a Type 1 SPD.

Type 2

This SPD can pre­vent the spread of over-­voltages in an elec­trical in­stall­a­tion and pro­tects equip­ment con­nec­ted to it. It prevents the spread of overvoltage’s in the electrical installations and protects the loads. This device would nor­mally be in sub-dis­tri­bu­tion boards and in the primary dis­tri­bu­tion board if there was no re­quire­ment for a type 1 device

Type 3

These SPDs have a low dis­charge ca­pa­city. They must there­fore only be in­stalled as a sup­ple­ment to Type 2 SPD and in the vi­cin­ity of sens­it­ive loads. Type 3 SP­D’s are char­ac­ter­ised by a com­bin­a­tion of voltage waves.


Unfortunately, SPD’s usually have a lifespan of one surge and cannot be reset. Most SPDs have an in­dic­a­tion win­dow that show that they are op­er­a­tional. If the in­dic­ator is green they are provid­ing pro­tec­tion. If they are red, then they have reached ‘end of life’ and will need re­pla­cing. Of­ten there is a re­place­able cart­ridge which can simply be with­drawn and re­placed with a new op­er­a­tional device.

If you’re unsure as to whether your SPS has reached the end of its life, then you should always contact a qualified electrician to check. You will normally still have a power if your SPD has come to end of its life. You will, however, no longer be protected.

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