EMC implementation - Implementation of surge arresters
Connections
They must be as short as possible. In fact, one of the essential characteristics for equipment protection is the maximum level of voltage that the equipment can withstand at its terminals. A surge arrester with a protection level suitable for the equipment to be protected should be chosen (see Fig. 16). The total length of the connections is L = L1 + L2 + L3. It represents an impedance of roughly 1 µH/m for high frequency currents.Application of the rule
[math]\displaystyle{ \Delta U=L\frac{di}{dt} }[/math]
with an 8/20 µs wave and a current of 8 kA leads to a voltage of 1,000 V peak per metre of cable.
[math]\displaystyle{ \Delta U=1.10^{-6}\, \times\, \frac{8.10^3}{8.16^{-6}}=1,000 V }[/math]
Fig. R16: Surge arrester connection: L < 50 cm
This gives U equipment = Up + U1 + U2.
If L1 + L2 + L3 = 50 cm, this will result in a voltage surge of 500 V for a current of 8 kA.
Wiring rules
- Rule 1
The first rule to be respected is not to exceed a distance of 50 cm when connecting the surge arrester to its disconnection circuit-breaker. The surge arrester connections are shown in Figure R17.
Fig. R17: SPD with separate or integrated disconnector
- Rule 2
The outgoing feeders of the protected conductors must be connected right at the terminals of the surge arrester and disconnection circuit-breaker (see Fig. R18).
Fig. R18: Connections are right at the SPD's terminals
- Rule 3
The phase, neutral and PE incoming wires must be tightly coupled to reduce the loop surfaces (see Fig. R19).
Fig. R19: Example of wiring precautions to be taken in a box (rules 2, 3, 4, 5)
- Rule 4
The surge arrester's incoming wires must be moved away from the outgoing wires to avoid mixing the polluted cables with the protected cables (see Fig. R19).
- Rule 5
The cables must be flattened against the metallic frames of the box in order to minimise the frame loops and thus benefit from a disturbance screening effect. If the box is made of plastic and the loads particularly sensitive, it must be replaced by a metal box.
In all cases, you must check that the metallic frames of the boxes or cabinets are frame grounded by very short connections.
Finally, if screened cables are used, extra lengths which serve no purpose ("pigtails"), must be cut off as they reduce screening effectiveness.
