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Which Hochiki CHQ modules need an external 24 V PSU?

CHQ-DZM(SCI) and CHQ-DSC2(SCI) need an auxiliary 24v DC supply (The CHQ-ISM conventional module used for an intrinsically safe sounder installation, also requires a 24v supply)

Applies to

  • Products / Models: CHQ-DZM(SCI), CHQ-DSC2(SCI) and CHQ-ISM (including /DIN and /M variants)
  • Firmware / Version: Not applicable
  • Region: Not applicable
  • Standards: BS EN 54-4 (power supply equipment, where an auxiliary supply is used), BS EN 54-17 (short-circuit isolators), BS EN 54-18 (input/output devices)

Direct answer

Whether a loop device needs an external power supply (PSU) depends on the module. CHQ-SZM2(SCI), CHQ-DIM2(SCI), CHQ-DRC2(SCI) and CHQ-PCM(SCI) for example, run entirely on loop power, including the CHQ-DRC2(SCI)'s two independently controlled relay outputs. CHQ-DZM(SCI), which drives a conventional detection zone, and CHQ-DSC2(SCI), which drives conventional sounder circuits (and the CHQ-ISM for intrinsically safe sounder installations), need a separate auxiliary 24 V DC supply. Confirm this against the specification sheet for the exact ordering code before designing the circuit.

Can the device be powered from the loop?

  • Whether an external power supply is needed depends on the module. Loop-powered modules (for example CHQ-SZM2(SCI), CHQ-DIM2(SCI) and CHQ-DRC2(SCI)) take all of their operating power from the ESP loop. Modules that drive a conventional detection zone (CHQ-DZM(SCI)) or conventional sounder circuits (CHQ-DSC2(SCI)) and CHQ-ISM require a separate auxiliary 24 V DC supply. Always check the "Requires an auxiliary supply" line on the specification sheet for the exact ordering code before design.
  • Loop power is sufficient for volt-free relay outputs on loop-powered modules such as CHQ-DRC2(SCI). An auxiliary supply is needed where the module powers something outside the loop — a conventional detection zone (CHQ-DZM(SCI)) or a switched voltage output that has to deliver current to a load.

Powering recommendations and wiring notes

  • Only fit an external PSU where the module needs one, for example a CHQ-DZM(SCI) driving a conventional detection zone, or a CHQ-DSC2(SCI) and CHQ-ISM driving conventional sounder circuits. Take the required voltage and current from the specification sheet for the exact ordering code, and use power supply equipment that conforms to BS EN 54-4, not a general-purpose 24 V DC supply.
  • Line continuity is a CHQ-DZM(SCI) feature. It applies to the conventional detection zone, not to the addressable ESP loop.  For example, CHQ-SZM2(SCI) zone has no line continuity option at all and takes the TE-RH-E Zener EOL only, so any manual call points on a CHQ-SZM2(SCI) zone must be wired first. On a CHQ-DZM(SCI), line continuity keeps the rest of the zone working when a detector head is removed from its base, and it depends on two things together: bases that contain an in-line Schottky diode (for example YBN-R/6SK or YBO-R/6RS) and the zone set to active or resistive EOL mode with the optional LCMU (6K8). If the Zener EOL device (TE-RH-E) is selected instead, the zone has no line continuity option, so any manual call points must be wired first on the zone. The intrinsically safe and SIL variants of the CHQ-DZM support no line continuity option at all, so on those modules any manual call points must be wired at the beginning of the zone.
  • Some device variants are supplied with short-circuit isolation built in; select the appropriate variant if short-circuit isolation is required for your installation.

Practical steps

  1. Check the "Requires an auxiliary supply" line on the specification sheet for the exact ordering code, and fit an auxiliary 24 V DC supply only where that module needs one — for example a CHQ-DZM(SCI) driving a conventional detection zone.
  2. If line continuity is required on a conventional zone, fit bases with the correct in-line Schottky diode and set the zone to the active or resistive EOL mode specified for that zone.
  3. If your installation requires short-circuit isolation, choose the device variant that includes short-circuit isolation.

Troubleshooting tips

  • If the zone or loop does not return after a reset, check for a short-circuit indication on the module's amber LED and on the panel before changing any wiring.
  • If you see unexpected loss of continuity on a conventional zone when a detector head is removed, verify that the base has the correct in-line Schottky diode and that the zone is set to active or resistive EOL mode.

For the EOL value to fit on each input or zone, see What EOL resistor value do I need for a Hochiki monitored input or conventional zone?

When to contact technical support

  • You are not sure whether a specific module needs an external PSU or can run on loop power alone.
  • A conventional zone reports intermittent or open-circuit faults after the correct EOL option has been fitted and checked.
  • The module's amber LED indicates a short-circuit fault that persists after checking the wiring.

Safety note

This article is general guidance for Hochiki Europe products. Design, installation, commissioning, maintenance and modification of fire detection and alarm systems should be carried out by, or under the responsibility of, a competent person in accordance with the applicable standards (for example BS 5839-1 in the UK). Statutory requirements imposed by the relevant enforcing authority also apply. Always refer to the current product manual and data sheet for the exact model before carrying out any work.

References

  • CHQ Modules (Inputs) Installation Instructions, 2-3-0-778
  • CHQ-DZM(SCI) Specification
  • CHQ-DZM(SCI) Installation Instructions, 2-3-0-1731
  • CHQ-DRC2(SCI) Installation Instructions, 2-3-0-2802
  • CHQ-DRC2(SCI) Specification
  • CDX Range Technology Guide

Last reviewed: 2026-10-16 — Reviewed by: Hochiki Europe Technical Support