DH-55 Metal Door Contact – NC & NO/NC 3-Wire Magnetic Sensor

• The DH-55 Metal Door Contact is a durable wired magnetic sensor designed for demanding security applications. It features a reinforced metal housing that withstands impact and stress, making it suitable for commercial entrances, warehouse doors, rolling shutters, gates, and industrial areas where plastic contacts fail.

  Two wiring variations are available:

  • NC Contact – Circuit remains closed when door is shut and opens when door opens.

  • NO/NC 3-Wire Contact – Provides universal configuration to support both wiring needs using a single sensor.

  The strong magnetic trigger ensures stable detection even with vibration or heavy metal doors. It integrates easily with alarm panels, access control systems, CCTV alarm inputs, and smart relay modules.

  Designed for high-security environments, the DH-55 offers long-term reliability and dependable intrusion detection performance.

A metal door contact with three wire flexibility

The DH-55 is a surface mount magnetic contact built specifically for metal doors, and supplied in a three wire configuration offering both normally closed and normally open outputs. That combination is what sets it apart: one sensor can signal an alarm panel and, at the same time, drive a second device such as a relay, indicator, or automation input.

Why metal doors need a purpose built contact

Steel and aluminium distort magnetic fields, which is why an ordinary plastic door contact often behaves unpredictably when fitted to a metal door or frame. It may fail to detect closure, or report the door open when it is shut. A contact designed for metal doors accounts for this, and is housed to survive the knocks that steel doors and their surroundings tend to deliver.

Where it is fitted. Factory and warehouse steel doors, godown entrances, metal framed commercial doors, plant rooms, and any hard working door where a standard plastic contact has already cracked or proved unreliable.

Understanding the three wire NO and NC output

A two wire contact gives you one circuit. This three wire version provides a common plus both a normally closed and a normally open contact, so you can choose either behaviour, or use both at once. The practical value is flexibility: wire the NC side into your intruder alarm zone for security, and use the NO side to trigger something else, such as a relay module switching on a light, an indicator lamp at a guard desk, or an input to a building automation controller. If you only need a simple alarm connection, use the NC pair and leave the third wire unused.

Why alarms use the NC side. With a normally closed circuit, current flows continuously while the door is shut. Opening the door breaks the circuit and triggers the alarm, and so does cutting the cable. That means an intruder cannot quietly defeat the sensor by snipping the wire, because doing so produces the same alarm as opening the door. This is the standard configuration for security work.

Mounting on steel correctly

Fit the sensor body to the fixed frame and the magnet to the door, directly opposite one another rather than at an angle. If detection is inconsistent, mount both halves on thin non metallic spacers so they sit slightly proud of the steel, which reduces the interference the metal causes. Keep the closed gap comfortably inside the rated operating distance rather than at its outer limit, leaving margin for the door settling over time.

Alignment causes most faults. When a door contact starts misreporting, alignment has usually drifted rather than the sensor having failed. Heavy steel doors drop on their hinges with use and frames move with heat. Fit the halves centred, and check again a few weeks after installation once the door has settled into its daily pattern.

Cabling. Run cable to the fixed frame side, never to the moving leaf, and keep it inside conduit. On an industrial door an exposed sensor cable is both a maintenance problem and an obvious point of attack. Label all three cores at both ends, because a three wire contact is considerably easier to work on later if someone has recorded which core is common, which is NC, and which is NO.

No batteries, no internet dependency. Being wired, the contact draws power from the panel or circuit it serves, so there is nothing to replace and nothing to go flat unnoticed. It needs no WiFi, router, or internet connection, so the door remains monitored through load shedding as long as the panel has its backup battery. On a commercial door protecting stock or plant, that independence is the entire argument for wired over wireless.

Choosing between our door contacts

See our full range of door sensors and relay modules to compare recessed, surface mount and rolling shutter options. Use this DH-55 for metal doors, and where you want both NO and NC outputs from one sensor. Use the recessed DH-35 or DH-36 where you want an invisible, tamper resistant contact on a quality wooden door. Use the DH-58 rolling door contact for steel shutters, where the gap is wide and inconsistent. Use the DH-32 surface mount for straightforward retrofits. Use a wireless Tuya sensor where you want phone alerts and cannot run cable.

Do I have to use all three wires? No. Use the common and NC pair for a standard alarm connection and leave the NO core unconnected and insulated.

Will it work on an aluminium framed glass door? Yes, this is a common application, using spacers if detection proves marginal.

Does it need its own power supply? No. It is powered by the alarm panel or circuit it connects to.

Can it be used outdoors? Fit it in a sheltered position. For fully exposed installations tell us and we will recommend a suitably rated alternative.

Buying from ZZ Marts. Available at our Karachi store and Lahore store, and typically installed alongside fire and burglar alarm systems or access control systems. In stock at our Karachi (Saddar) and Lahore stores with nationwide delivery, supplied by Zam Zam Traders, trading in security hardware since 2005. We also stock recessed reed contacts, rolling shutter contacts, wireless Tuya door sensors, alarm panels, sirens, relay modules, and cabling. Tell us your door type and what you are connecting to, and we will specify the whole installation rather than just the sensor.