Remote Signal Terminal Of Surge Protection Device

Publish Time: 07/25 2025 Author: Site Editor Visit: 211

Introduction

Remote signal terminal (Dry Contacts) of a Surge Protective Device (SPD) are primarily used for remote monitoring of the SPD's operational status. When the SPD deteriorates or fails, it outputs a discrete switch signal. If clients wish to remotely observe SPD degradation via a computer system, these signaling contacts can be integrated into a monitoring system for real-time alerts. As illustrated in the diagram, the remote signaling contacts typically protrude as a small green terminal on one side of the SPD, making them easily identifiable.

Composition

The COM, NC, and NO terminals of the remote signaling contacts constitute a status switching circuit:

COM (Common Terminal): The common connection point for signal paths.

NC (Normally Closed):

During normal SPD operation, NC and COM maintain a closed (closed-circuit) connection.
Upon SPD failure, the NC-COM circuit transitions to an open-circuit state.

NO (Normally Open):

During normal SPD operation, NO and COM remain in an open-circuit state.
Upon SPD failure, the NO-COM circuit forms a closed (closed-circuit) connection.

Working Principle

Under normal SPD operation (the thermal trip device not activated), the NC-COM terminals maintain a closed circuit, while the NO-COM terminals remain an open circuit . Upon SPD failure (e.g., damage from lightning strikes or overheating), the thermal trip device activates, causing the NC-COM circuit to open and the NO-COM circuit to close. This state transition generates a discrete switch signal to the monitoring system (e.g., PLC or Building Automation System), triggering an alarm.

Remote Signal Terminal Of Surge Protection Device

Application Selection

Scenario	Recommended Contact	Rationale
Requires "open-circuit triggers alarm" during faults	NC	Under normal SPD operation, NC-COM maintains a closed circuit (closed-circuit), while upon SPD failure, it transitions to an open-circuit state. This is compatible with systems configured to interpret circuit interruption as an alarm signal (e.g., critical power SCADA systems).
Requires "closed-circuit triggers alarm" during faults	NO	Under normal SPD operation, NO-COM remains an open circuit, while upon SPD failure, it forms a closed circuit (closed-circuit). This suit systems designed to activate alerts upon contact closure (e.g., building automation systems).

Wiring Precautions

Grounding for EMI Suppression：The signal cable shield layer must be single-point grounded (at either the SPD or equipment terminal) to prevent electromagnetic interference from causing false alarms.
Cable Length Restriction：Recommended maximum length: ≤50 meters. Exceeding this limit may cause signal attenuation; install signal repeaters if longer distances are required.
Compatibility Verification：Ensure the monitoring device's input voltage/current matches the SPD contact rating (typically ≤24V DC, 100mA). Exceeding these limits may damage the monitoring circuit.
Safety Compliance：Insulate exposed conductors with electrical tape after termination.

Avoid parallel routing with power cables (minimum separation ≥20cm) to reduce induced noise and arc risks .

Application

Mday SPD support optional remote signal terminals. Mday's remote signal terminal interface is a universal RS485, which can be seamlessly integrated into various systems.

System：