What is a surge protection device (SPD)? Why your EV charger and home need one

Short answer: A surge protection device (SPD) protects equipment by diverting the short, thousands-of-volts voltage spikes caused by lightning or grid switching to earth. Circuit breakers and RCDs cannot stop these spikes; they protect against overcurrent and leakage current. An EV charger, the car and the electronics in your home are all sensitive to surges. In ENGN's charger installations in Bursa, an SPD is standard together with a 40 A type A RCD and a 40 A circuit breaker. Local rules apply.

Where do surges come from?

• Lightning: a strike on the building, a nearby pole, a tree or an overhead line creates a very short but very high voltage on the wiring and the earthing. Even a nearby strike induces a surge in the cables.
• Grid switching: the network operator switching lines, and large motors, compressors or welding machines starting and stopping, produce smaller but frequent surges.
• Faults: short circuits on the network or a lost neutral. A lost neutral causes a long-lasting overvoltage, which an SPD does not handle; an over/under-voltage relay does (see below).

These surges last microseconds. They may look like a flicker of the lights, yet they are enough to damage the charger's control board, the car's on-board charging electronics, a router, a boiler or an appliance's circuit board.

How does an SPD work?

At normal voltage the SPD has a very high resistance and is effectively out of circuit. When the voltage exceeds a threshold it becomes conductive within nanoseconds and diverts the surge current to the earthing conductor; once the surge has passed it returns to its insulating state. That is why an SPD is connected between the phase (and neutral) and earth. Without earthing, or with high earth resistance, the surge current has nowhere to go and the SPD cannot do its job. Measure the earthing first and correct it if needed.

Type 1, type 2 and type 3

SPDs are classified by their test class under IEC/EN 61643-11:

• Type 1 (T1): tested against direct lightning current (10/350 µs wave, Iimp rating). Installed at the main board where the building has a lightning protection system or is supplied by an overhead line.
• Type 2 (T2): tested against indirect lightning and switching surges (8/20 µs wave, In rating). The most common type in homes and businesses and the basic protection on an EV charging circuit.
• Type 3 (T3): fine protection placed right next to sensitive equipment (combination wave, Uoc rating). Its discharge capacity is low, so it is never used alone; a type 2 must be installed upstream.

Combined type 1+2 devices carry lightning current at the main board while giving a type 2 level of protection.

Ratings on the label: Uc, Up, In, Imax, Iimp

• Uc (maximum continuous operating voltage): the voltage the SPD can withstand continuously. It is chosen above the supply voltage; on a 230/400 V network at least 1.1 × 230 V phase to neutral, typically about 275 V in catalogues.
• Up (voltage protection level): the voltage left across the SPD during a surge, i.e. what reaches the equipment. Lower is better, and it must be below the impulse withstand of the protected equipment. In 230/400 V installations, fixed installation equipment is in the 4 kV category, ordinary appliances 2.5 kV and sensitive electronics 1.5 kV.
• In (nominal discharge current): the current a type 2 can carry repeatedly with an 8/20 µs wave (e.g. 5, 10, 20 kA).
• Imax (maximum discharge current): the highest current it can carry once. Catalogues highlight it, but selection is based on In and Up.
• Iimp (impulse current): the 10/350 µs lightning current rating of a type 1.

Where in the installation?

1. Main board (building intake): protects the whole installation. Type 1 or 1+2 if there is a lightning protection system or overhead supply, otherwise type 2.
2. Charger sub-board or the circuit to the charger: if the charger is far from the main board (roughly more than 10 metres), the surge can build up again along the cable, so a second type 2 is fitted close to the charger. Car parks, gardens and shared garages often need this.
3. Next to sensitive equipment: type 3 where needed (socket or DIN-rail type).

Connection leads are kept as short as possible: the recommended total length of the phase–SPD–earth connection is 0.5 metres. Long, looped leads add their own inductive voltage and reduce the SPD's effect.

Earthing system and wiring scheme

The wiring scheme depends on the earthing system. In a TN-S system a module is connected between each phase, the neutral and the protective conductor; in a TT system a 3+1 arrangement is common (three modules phase to neutral, a spark-gap module neutral to earth). An SPD wired with the wrong scheme either does not protect or trips the RCD. The correct scheme is decided in the design.

Back-up protection

At the end of its life, or after a very large surge, an SPD can short-circuit. To prevent fire or a total outage, a fuse or breaker of the value stated in the manufacturer's catalogue is installed upstream. Some products have this built in; the catalogue says so.

Lifetime and indicator

Every surge wears an SPD a little. Most DIN-rail SPDs have an indicator window: green means working, red means the module must be replaced. With plug-in modules only the module is changed. Check the indicator after a thunderstorm and once a year. Models with a remote signalling contact are useful in shared buildings and businesses.

What an SPD does not do

• It does not replace an RCD: personal protection comes from the RCD. Why a charging circuit uses a type A RCD is covered in a separate guide.
• It does not replace a circuit breaker: overload and short-circuit protection is the breaker's job.
• It does not stop long-lasting overvoltage: a lost neutral or the supply sitting at 250–260 V for a long time needs a separate over/under-voltage relay, which disconnects the circuit outside the limits and reconnects when the voltage recovers. In ENGN installations this is optional and quoted separately.
• It does not protect the building from a direct strike: that is the job of an external lightning protection system; the SPD protects the internal wiring.

Why an EV charger needs one

A home EV charger contains communication, metering and control electronics and is often installed in a garage, car park or garden at the end of a long cable. Long runs are more exposed to surges. During charging the car is connected to the same circuit, and its on-board charger sees the surge directly. The installation standard IEC 60364-7-722 requires overvoltage protection for charging points accessible to the public; at home it may not be mandatory, but an SPD costs very little compared with the charger and the car. That is why we fit it as standard rather than as an option.

ENGN's protection package in Bursa

In our installations each charging point is supplied by its own circuit, and a 40 A type A RCD, a 40 A circuit breaker and an SPD are standard. The SPD type and location (main board or charger sub-board), the earth measurement and the cable size are determined during the site survey and written into the design. Our installation service is available only in Bursa, Turkey; the survey is paid and the fee is deducted from the written quote if you accept the work, and every installation is carried out with an electrical design (project).

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• → Earthing explained
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