SPDs should be installed on the load sides of the service entrance, generator, and transfer switch, where shown at Positions SPD 1, SPD 2, and SPD 3, respectively, in the figure below. An existing service entrance SPD located near the transfer switch meets the intent of SPD 1. SPD 2 protects the generator. SPD 3 protects downstream loads and is arguably the most important SPD in this context.
Because control and monitoring equipment are also susceptible to electrical transients, any data and control circuits leading from the ATS and generator should also be protected by SPDs, where shown at the positions of SPD 4 and SPD 5. Longer cable runs should include SPDs at each end. This is particularly important when conductors enter facilities from outdoor locations. Select SPDs and voltage ratings based on the communication protocol.
Because Type 1 SPDs contain UL-required safety features that can simplify installation, Type 1 units should be considered for each location.
The SPD MCOVs should provide sufficient ‘voltage headroom’ to accommodate voltage fluctuations. For typical 480Y/277V Grounded Wye systems with adequate voltage regulation, MCOVs of 320V for Line-to-Neutral, Line-to-Ground, and Neutral-to-Ground modes, and 552V for Line-to-Line modes should be considered. For 480Y/277V Grounded Wye systems having questionable voltage regulation, or for 480V ungrounded systems, use 552V. For other voltage and grounding applications, obtain recommendations from the manufacturer’s application guide or customer support team.
SPDs with 20kA I-nominal discharge current ratings offer the greatest robustness, and are well-suited for locations near service entrances and ATS equipment.
To facilitate service, each SPD should be equipped with an integral disconnect switch or installed with a circuit breaker or on-off safety switch.
Published for:ASCO Power Technologies USA