How to size surge protection for cost-effective, optimised performance
Panel design is an art. A well-designed panel strikes a balance between form, function, and cost. This can be difficult considering that an attempt to do so is an attempt at achieving the best of those three worlds.
This article teaches panel designers how to determine the correct size of surge protection devices (SPDs) so that the system’s surge protection will achieve that balance in order to be both complete and cost effective.
Surge protection devices
SPDs reduce surge voltages to a level that the power distribution system and the connected equipment can handle. SPDs are primarily rated by how much of a surge current magnitude they can handle, and how well they limit the voltage when conducting that surge current.
SPDs are not one-size-fits-all devices, nor do the size of their kA rating (the surge rating) go hand in hand with the size of the panel. In other words, you should not select an SPD with a maximum kA rating just because the panel is large.
Modes of protection
There are four ways to connect MOVs:
• Line to Neutral (L–N)
• Line to Ground (L–G)
• Neutral to Ground (N–G)
• Line to Line (L–L)
L–N and L–L are appropriate for installations that are close to the service entrance. At the service entrance, the neutral and ground are bonded together, so the installation of an SPD from L–N would cause the connections from L–G and N–G to become redundant.
When far from the service entrance, N–G and L–G connections are optimal because the N–G is further from the neutral-ground bond connection. Modes can be directly or indirectly connected. By directly connecting two SPDs L–N, you also get an indirect L–L connection.
Characteristics of good surge protection
DIN-rail SPDs reduce the number of components, along with their associated costs. An SPD that does not require additional components to be built into the panel’s design is a big win in a compact size. This streamlines the facility’s inventory and simplifies its allocation of parts. This also helps make the SPD more compact. Compact SPDs enable flexibility in the panel design.
Panel designers should look for SPDs that:
• have a 20-kA nominal interrupting rating, which is the highest nominal interrupting rating per UL testing; and
• have a high SCCR (to comply with NEC 285.6)
Other important features include:
• An ability to clamp and withstand high-energy transients. This is important because it prevents disruption, downtime, and degradation or damage to equipment.
• Interlocking tab mechanism. This secures the module so that it can withstand vibration.
• A compact size. Small SPDs provide flexibility in the panel design.
• A visual life indicator. A quick visual is important because it determines the module replacement status, which avoids a loss of protection and saves time.
• Pluggable. A pluggable module makes the device’s replacement fast and simple. This will minimise the maintenance and downtime, and eliminate the need for tools.
• Thermal protection. Thermal protection eliminates catastrophic failure.
• An IP2+ rating. When the first number of the IP rating (such as IP20, IP21) is 2, the device will protect against solid objects up to 12mm (such as a person’s fingers).
How to select the correct SPD for each type of electrical system
An SPD that is the incorrect size for the system is a waste of money because it is either an unnecessarily high level of surge protection for the system (which is the best-case scenario) or provides inadequate protection and will have a short lifespan.
To determine which SPD should be used for the system, you should know the reference to ground and the type of voltage present. You should size the SPD to the voltage based on the type of system in use. This is based on the secondary side of the upstream transformer, not how the load is connected.
An SPD’s maximum continuous operating voltage (MCOV) is the maximum voltage that the SPD can withstand before it starts to clamp. The MCOV must be larger than the reference to ground for it to provide complete protection. Therefore, if the reference to ground is unknown, choose an SPD with an MCOV greater than the system’s L–L voltage for protection against the worst-case scenario.
An SPD’s clamping voltage should be a minimum of 15% higher than the circuit’s peak working voltage. The Littelfuse SPD2 series begins clamping between 15% and 2% of the working voltage, depending on the model.
However, there are repercussions for selecting an SPD that is inappropriately sized for the system:
• An MCOV that is too low for the system will enable the system’s susceptibility to temporary over-voltages. It also decreases the SPD’s life expectancy by burning out after one overwhelming power surge or by slowly degrading after repeated smaller surges.
• An MCOV that is too high for the system will chop off the surge and cause additional damage to the equipment.
The SPD will still work and provide some level of protection. However, either the SPD’s lifespan or its level of protection will not be as good as it could have been, had they selected on the SPD size based on the system’s reference to ground.
Determine the reference to ground
The reference to ground determines which product is best for the panel. The most common types of electrical systems that have a reference to ground are:
• 3-phase, 3-wire systems
• 3-phase, 4-wire systems
• 3-phase, 3-wire corner-grounded delta systems
• Ungrounded delta systems
• Impedance-grounded wye systems
• Resistance-grounded wye systems
• Single-phase (also known as split-phase) systems
• High-leg grounded delta systems
The formula to determine the reference to ground will depend on the type of system.
For example, to find the reference to ground for single-phase systems, divide the line-to-line voltage by two. For a 3-phase, 4-wire system, divide the line-to-line voltage by the square root of three.
To access a full guide containing information about how to find the reference to ground and optimal SPD size for almost every type of system, go to Surge Protection Device Sizing for Industrial Control Panel Design.
Knowledge is power. Theoretically, any size SPD can be used in a system. Panel designers that know the system’s reference to ground can provide a level of optimisation that will provide systems with long-lasting protection at a cost-effective price point.
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