FAQ: How do you correctly size a surge protective device (SPD) for your DC power system?

Example: Lightning strikes or irregularities on the utility grid can create voltage spikes beyond the power equipment’s maximum voltage ratings.

A surge protective device (SPD) is designed to prevent higher energy voltage peaks from reaching sensitive equipment and thus potentially causing damage.

If properly designed, how does an SPD work in a DC system?

• Excess voltage (beyond the rating of the equipment) is prevented from building up by controlled energy discharge between the affected DC or AC conductors.
• If a ground connection is present on the SPD, the SPD also monitors voltage differential between ground and the other conductors. If necessary, energy is discharged to prevent excessive voltage differences such as in a surge event. For this to work properly, the path to ground must be of low resistance.
• SPDs cannot protect from prolonged over-voltage for multiple seconds or minutes. This must be prevented by correct system sizing.

Steps to ensure your equipment will not get damaged when a surge in voltage takes place:

1. Make sure your system and SPD has a good, low-resistance connection to ground.
2. Match the SPD to the inputs of your power conversion equipment you want to protect by ensuring the “Uc” voltage in the SPD datasheet is at or just slightly (preferably 0 to 10 V) above the maximum continuous voltage on the conductors to be protected, or the maximum voltage rating of the power equipment connected. If the SPD’s “Uc” rating is well above the maximum voltage rating of the power equipment connected, it can no longer effectively protect from voltage surges. The SPD will protect devices or equipment by activating well above the maximum continuous operating voltage “Uc” and will not interfere at voltages below “Uc”.
3. Phocos recommends protecting at least the PV input of the charge controller or inverter/charger and if using a public electric grid, protect the AC input as well.
4. If used on the PV conductors, ensure the SPD is rated for DC voltages, if used on the AC input, ensure the SPD is rated for AC voltages.