Understanding Low Inrush Current Relay Coils and Why They Matter

Relays are a core component in building automation systems, acting as the bridge between low-voltage control circuits and the high-power loads that actually do work. In many applications—from lighting and HVAC to motors and pumps—relays switch loads on and off based on signals from controllers, sensors, or schedules.

While most relays are designed to handle a wide range of typical switching tasks, certain applications introduce a challenge that standard relay coils aren’t ideal for: inrush current.

Motors, transformers, large capacitive loads, and some types of lighting fixtures can draw several times their normal operating current for a fraction of a second when first switched on. That temporary surge can stress relay coils and contacts, lead to nuisance trips, reduced service life, or in extreme cases, premature failure.

Low inrush current relay coils are engineered to address these exact conditions by limiting how much current the coil itself draws and how it responds when energized.

What “Low Inrush Current” Really Means

When a relay coil is energized, it needs enough current to build a magnetic field strong enough to pull the internal armature and close the contacts. A standard relay coil will accept whatever current the control circuit supplies, often resulting in a relatively high draw at the moment of energization. In many BAS installations, this works fine, because controllers and power supplies are sized with that in mind.

But in installations where control transformers, small power supplies, or sensitive electronics are involved, that initial draw can be problematic. A low inrush current coil is wound or designed so that it limits the peak current required to actuate the relay. The reduced demand at energization makes it easier for smaller power sources—or sources that are already heavily loaded—to reliably drive the relay coil without sagging voltage or triggering protective limits.

Importantly, the “low inrush” specification typically refers to the coil’s current draw, not the relay contacts. You still need to specify contact ratings appropriate for the load you are switching. The low inrush designation simply means the control side of the relay is easier to drive under constrained power conditions.

When You’d Choose a Low Inrush Coil

Not all relay applications require a low inrush coil, but there are several common cases where this feature provides real value.

• Constrained Control Power: When a relay coil is powered from a small Class 2 transformer or a power supply already near capacity, a low inrush coil helps avoid overloading that source at the moment the relay energizes.
• Sensitive Electronics: In spaces with sensitive control electronics, minimizing electrical disturbances improves system reliability and reduces the chances of resets or spurious behavior.
• Large Relay Counts: In control panels with many relays driven from the same power supply, lowering the peak draw of each coil reduces total demand spikes when multiple relays operate simultaneously.
• Retrofitted Panels: Older control panels retrofitted with additional relays may not have headroom in existing transformers or power supplies. Low inrush coils make adding new relays less disruptive.
• Battery-Backed or UPS Systems: When control power is backed up by batteries or UPS devices, limiting inrush current can improve battery life and reduce instantaneous stress on backup sources.

In these scenarios, a low inrush relay coil lets designers balance system performance with practical installation and service considerations.

How Functional Devices Addresses Inrush Challenges

The engineers at Functional Devices have worked to create a RIB relay that is suited for applications requiring a relay device with low or limited inrush current on the relay coil. A relay coil with low or limited inrush current is helpful when the device or controller sending the voltage signal to the coil is sensitive to how the relay coil draws in that current. If the current is drawn too fast and at a high amount, then it could be detrimental to the controller.