Correct Installation of Panel Surge Protective Devices

It is very important to follow the manufacturer’s installation instructions. Pay particular attention to fusing 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 panel-mounted transient voltage SPDs is affected by the connecting leads. Both the wire size and length used to connect the SPD will influence its performance.

Connecting Leads:
Transients have fast-rising wave-fronts, with typical rates of rise of current (di/dt) of 100 amps per microsecond. The self-inductance (L) of the connecting wiring is significant (0.1 (H per foot) and can hinder suppression of high voltages during passage of the wave- front. 

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. Typical Installation Procedure

Typical Manufacturer’s 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 TVSS panel 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.

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