Circuit protection
Breaker Accessory Control-Power Calculator
Estimate the control-supply demand of breaker accessories — shunt trips, undervoltage releases, motor operators, and auxiliary contacts — so the control transformer, PSU, or DC supply is sized for both the momentary pickup surge and the steady holding load. Enter one accessory at a time and add the results.
Pickup surge versus holding load
Accessory coils draw two very different currents. The pickup (inrush) VA is a brief surge when the coil energises — it sizes the supply’s peak capability and sets the voltage dip other devices will see. The holding VA is the continuous draw once energised — it sizes the steady thermal load. A supply that meets the holding load but not the pickup surge will drop out or fail to operate the device.
Typical accessory demand (illustrative)
| Accessory | Pickup | Holding | Note |
|---|---|---|---|
| Shunt trip (MX) | ~100–300 VA | 0 | Momentary; energised only to trip |
| Undervoltage release (MN) | ~200–350 VA | ~5–10 VA | Continuously energised to keep the breaker closable |
| Motor operator | ~300–600 VA | 0 | Draws during the spring-charge cycle only |
| Auxiliary / alarm contact | signal | ~0 | Dry contact; negligible coil load |
Values vary widely by series, frame, and AC or DC coil — always use the datasheet figure. Where several accessories can energise together (for example a motor operator recharging while an undervoltage release is held), add their pickup VA to size the worst-case surge.
Frequently asked questions
Shunt trip or undervoltage release for remote tripping?
A shunt trip trips when it is energised — it needs a healthy supply to operate. An undervoltage release trips when its supply fails — it is inherently fail-safe and also trips on supply loss. Choose by whether loss of the control supply should trip the breaker.
Why size the supply on peak, not holding?
The pickup surge sets the momentary current and the voltage dip. If the transformer sags below the coil’s operating band during pickup, the accessory may not pull in — even though it would hold fine afterwards.
Does a DC coil change the sizing?
DC coils have a different inrush profile and no power-factor effect, and battery or PSU supplies have their own limits. Use the DC VA/W figure from the datasheet and check the supply’s momentary current rating.
Match the accessory
Confirm the coil voltage, type, and series against the breaker, then select the device and its control supply.
Estimate for guidance only. Final accessory selection, coil ratings, and control-supply sizing must be verified by a qualified person against the manufacturer’s datasheet and the actual control-circuit design.