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How SPD’s Work

A look into the world of type 2 Surge Protection

What is a Type 2 Surge Protection Device?

There are three different classes of Surge protection devices that we commonly use in the United Kingdom. They are appropriately named, Type 1, Type 2 and Type 3.

As you would expect, each classification of SPD is designed to work at different stages of an electrical installation.

Type 1 SPD’s - These are recommended for specific use in the service sector and industrial buildings which are protected by a meshed cage or a lighting protection system. This type of SPD protects electrical installations against direct lighting encounters.

Type 2 SPD’s - The Type 2 SPD such as the VCPS2K is the primary protection system for all low voltage electrical installations in the U.K. Din rail mounted in the domestic consumer unit, it is designed to prevent over voltages spreading throughout the electrical installations. This protects both the devices installed in the consumer unit as well as the appliances within the home.

Type 3 SPD’s - These SPD’s have a very low discharge capacity and are therefore only installed as a supportive or supplementary protection device to an existing Type 2 SPD installation. Usually found on expensive appliances and equipment, such as certain fridge freezers, computers, TV’s etc. Some trailing sockets now also include this type of SPD.

How do Type 2 SPD’s work?

There is a lot of misunderstanding as to how T2 SPD’s actually function and understanding this will help educate your client, giving them better information so they can make an informed decision as to whether they should include one in their consumer unit.

A surge protection device (SPD) guards against the damage that sudden power surges from either end of the circuit can cause. It works by pulling the current from one outlet and passing it through to the devices you have connected to the surge protector. A surge protector contains a metal oxide varistor, also referred to as an MOV, which diverts any extra voltage to ensure devices receive a consistent power level.

The MOV works like a pressure-sensitive valve in a similar fashion as you would find a pressure relief valve on a water main. When the MOV detects high voltage levels, it reduces resistance. If voltage levels are too low, it increases resistance. It will kick in automatically to redirect excess voltage.

At normal operating voltages, the SPDs are in a high-impedance state and do not affect the system. When a transient voltage occurs on the circuit, the SPD moves into a state of conduction (or low impedance) and diverts the surge current back to its source or ground. This limits the voltage to a safer level. After the transient is diverted, the SPD automatically resets back to its high-impedance state. It is important to note, that all SPD’s do have a lifespan and will need to be monitored and maintained over a period of time.

MOV, GDT or Hybrid?

Simply put, MOV’s are able to handle a high voltage of overcurrent whilst a GDT works in a manner that provides a reduced amount of leakage. Individually, both are very effective protection devices, that conduct overcurrent through and dissipate this as milliseconds of heat.

Combing the two into a single device such as our Verso® VCPS2K, provides an optimum level of surge protection that is suitable for more than one type of earthing system.

Overcurrent protection?

Whilst you may feel that putting a short circuit across the mains is not a good idea, you must remember that SPD’s are designed to protect against transient voltage. This means the over voltage will be no more than a matter of milliseconds. However, it is important to ensure that when installing an SPD, you have some form of overcurrent device in place. You do not want to technically place a short circuit over the incoming supply of the consumer unit without added protection. In the event that a fault should occur within the SPD, an extremely large current will flow through the SPD continuously. This over a period of time would conduct enough heat to create a risk of fire. A simple 32A MCB will suffice as overcurrent protection to cut off the supply to the SPD in the event of this happening, protecting the consumer unit so it can operate normally without risk, in the event of a rare SPD failure.

Many brands provide a dedicated 32A MCB to this effect as we do here at Verso®, adding a layer of safety allowing to make maintaining these protection devices easier and compliant with the latest IET 18th edition wiring regulations.

Are SPD’s compulsory on domestic installations?

The wording in the regulations almost mandates fitting SPDs unless you perform a risk assessment to prove that you don’t require one. The formula needed to complete a risk assessment is complex to say the least, requiring an estimation of the cable lengths in the high and low voltage distribution networks supplying the installation. Due to this being a compulsory part of the assessment, many electrical professionals find it more cost effective in time alone to install an SPD.

However, the wording also excludes single residential units where the total value of the installation and equipment therein does not justify such protection. To be clear, this means that the electrical contractor has to convince the client to justify the additional cost of the kit.

This may be very difficult to do as a professional with SPD kits being inexpensive in todays market, and the average cost of a TV, Computer or white goods being at least ten times that of an SPD.

This may change in future amendments of the current 18th Edition, with amendment 2 due to be published this March (2022) and becoming mandatory in September 2022.


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How SPDs Work
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