It is very important to follow the manufacturer’s installation instructions. Pay particular attention to fuse or breaker requirements and lead lengths.
It is also important that the electrical distribution system be grounded and bonded per the National Electrical Code®. Failure to do so may result in damage to the surge protective device (SPD).
The performance of parallel-connected transient voltage SPDs is affected by the connecting leads. Both the wire size and length used to connect the SPD will influence its performance.
Transients have fast-rising wave-fronts. Typically the rate of rise of the current (di/dt) associated with surges can be 100 amps per microsecond or faster. The self-inductance (L) of the connecting wiring is significant (0.1 uH per foot) and can hinder suppression of high voltages during passage of the wavefront.
The voltage drop (V = L di/dt) across the connecting leads is added to the voltage across the suppression elements thus degrading the SPD’s performance by increasing the residual voltage.
Figure 1. SPD performance is a function of the connecting lead length.
Self-inductance of wiring is proportional both to its length and to the logarithm of its thickness. Halving the length of connecting wires halves the inductance, but the thickness would have to be increased tenfold to achieve the same effect. Stranded wires have greater effective thickness than equivalent sized solid conductors because of skin effect on the total surface area.
Thick, short, stranded connecting wires will give the best SPD performance. However, short length is much more important than large wire size.
Figure 2. Example of Installation Procedure
Locate the SPD as close as possible to the panel to be protected.
Drill and punch a hole in the SPD housing in a position to minimize the length of the connecting wires from the lugs of the SPD to the circuit breaker in the adjacent panel (or fused disconnect lugs).
Where possible, use a close-nippled connection with wires going directly to the first breaker at the top of a panel. This ensures optimum protection of all loads connected to the panel.
Use AWG #10 stranded wire or larger (which is readily available and easily installed) to connect between the SPD and the breaker panel. Avoid sharp bends and excess length in the wiring. Neat and tidy installations are not necessarily the most effective ones. Short direct connections are best.
SPDs should be connected through an appropriately rated circuit breaker not into the main lugs of the panel. Where circuit breakers are unavailable or impractical, a fused disconnect switch should be used to connect to the lines and facilitate servicing of the SPD.
NOTE: This example represents one of many acceptable ways to install hard-wired SPDs. Check with the manufacturer for suggested installation procedures.