A load relay is a relay used to switch an electrical load on or off. It enables a controller, programmable logic controller (PLC), or EMS to switch electrical loads based on measurements, schedules, and operating conditions.

Load Relay at a Glance

• A load relay converts an electrical control signal into a defined switching state.
• Loads can be switched based on factors such as PV surplus, grid consumption, a load limit, or the state of charge of a BESS.
• Small controller relays normally do not switch high-power loads directly. Instead, they operate a suitable power contactor.
• The electrical design and control logic must be suitable for the load type, switching frequency, and required behavior in the event of a failure.

How Does a Load Relay Work?

A control signal actuates the relay and opens or closes an electrical contact. The signal can be issued by a controller, PLC, or EMS. The higher-level control system first evaluates measurements and operating conditions and then uses predefined logic to determine whether the connected load should be activated or deactivated.

For measurement-dependent switching, different switch-on and switch-off thresholds, time delays, minimum operating times, and lockout periods are often used. This hysteresis and these timing conditions prevent the load from being switched on and off continuously when measured values fluctuate.

Relay contacts can be configured as Normally Open (NO) or Normally Closed (NC). An NO contact is open when not actuated, while an NC contact is closed. The appropriate configuration depends particularly on the state the load should assume if the control system or power supply fails.

A volt-free contact, also known as a dry contact, provides a switching contact without directly connecting the control circuit electrically to the switched circuit. The contact does not normally supply its own voltage. An external voltage or separate control circuit is therefore required. The device specifications determine which voltages may be switched and which insulation requirements are met. A volt-free contact must not automatically be considered to provide sufficient electrical separation for every application.

A small relay output on a controller is often designed for only limited voltages and currents. For higher-power loads, the relay therefore switches only the coil of a power contactor, which carries the actual load current.

Where Are Load Relays Used?

In PV systems, flexible loads can be activated automatically when PV surplus is available. Typical examples include heating elements, pumps, ventilation systems, cooling systems, and certain charging processes.

For load limitation, an EMS can disconnect less important loads before a defined grid consumption or power limit is exceeded. The load can be enabled again once sufficient power is available.

In off-grid and backup power systems, load relays can also provide automatic load shedding. Non-critical loads are disconnected according to defined priorities when the available generation or storage capacity is insufficient.

Additional switching criteria can include schedules, electricity prices, system conditions, or the state of charge of a BESS.

Technical Limitations and Requirements

The switching voltage, maximum current, contact rating, and permitted number of switching cycles must be suitable for the respective application. The rated power of the load is not the only relevant factor. Motors, transformers, and other inductive loads can cause high inrush currents and voltage spikes that place additional stress on relay contacts.

Solid-state relays or other power-electronic solutions may be more suitable for frequent or rapid switching operations. The manufacturer’s specified minimum operating times, lockout periods, and permitted switching frequencies for the load must also be considered.

Safety functions, motor protection, and electrical interlocks must not depend exclusively on higher-level EMS logic. Required protective functions must be implemented independently and in accordance with the specific system.

Controlling Load Relays with EcoPhi

EcoPhi can control load relays or downstream power contactors through suitable digital outputs or external I/O modules. This allows loads to be activated when PV surplus is available, disconnected to maintain a load limit, or managed according to priorities during off-grid operation.

The specific implementation depends on the switching capacity, type of load, required safety interlocks, and available inputs and outputs. Depending on the project, additional components, interfaces, or engineering services may be required.

Conclusion: Switching Loads Based on Operating Conditions

A load relay connects the control logic of a controller or EMS to an electrical switching contact. This allows loads to be switched on or off automatically based on measurements and operating conditions. For reliable operation, the electrical design, switching logic, and required behavior in the event of a failure must be defined for the specific project.

Frequently Asked Questions About Load Relays

Can a controller relay switch a load directly?

This depends on the electrical rating of the relay output and the characteristics of the load. For higher power levels or high inrush currents, the controller relay normally operates a suitable power contactor.

What is the difference between a relay and a power contactor?

Both components switch electrical circuits. Power contactors are typically designed for higher load currents and the repeated switching of industrial loads. However, whether a relay or power contactor is suitable depends on the load type, switching frequency, utilization category, and intended electrical service life.

Can a load relay regulate the power of a load continuously?

A conventional load relay generally provides only two states: on and off. Continuous power control requires a suitable communication interface, power controller, or another power-electronic solution.

What happens if the control system fails?

The resulting state depends on whether an NO or NC contact is used and on the electrical circuit design. The required safe state must be defined separately for each load and project.