Door Contact Selection: Gap Ratings and Roller Doors NZ
Essential Security Supplies
Door contacts look simple, but many nuisance alarms and incorrect door-held events originate in poor contact selection or alignment. A contact that works when held together on the bench may not operate reliably on a moving steel door, vibrating roller shutter or frame that flexes with seasonal changes. New Zealand installers need to understand operating gaps, magnet orientation, mounting substrate and the difference between a reed contact and a monitored locking output. This guide focuses on choosing the right contact for its actual door movement and on proving the installation under real conditions.
How door contacts detect position
A typical magnetic reed contact combines a switching element on one surface with a magnet on the moving surface. As the leaf moves away, the field changes and the contact state changes. This is a position indication, not a measurement of locking strength. A contact can show a closed leaf even if a bolt has not engaged. Conversely, an incorrectly aligned reed may indicate open while the door is physically shut. Access software must interpret the contact together with lock and request-to-exit signals.
Normal-open and normal-closed terminology can describe the switch in its unactuated state, which may be confusing when the installed door is shut. Confirm the manufacturer's definition with a meter and the magnet in both positions. Check how any end-of-line supervision is wired. A supervised zone can help distinguish certain wiring faults from legitimate state changes, but only with correctly selected resistance values and compatible panel programming.
Understand gap ratings without guessing
The advertised gap is a guide under the manufacturer's test conditions, not a guaranteed allowance for every material and geometry. Magnetic strength, magnet polarity, contact orientation, adjacent steel, door sag and mounting recess all influence the switching point. Two contacts with a similar headline rating can behave differently on steel and aluminium. For each SKU, use the current datasheet's specified make, break or operating gap and any stated substrate qualification.
Design for a reliable operating margin rather than installing exactly at the claimed maximum. A door may move a few millimetres in wind, under latch pressure or as hinges wear. Position the magnet so a properly closed leaf produces a stable state, yet opening the leaf produces a definite transition before a significant security gap exists. Do not silently substitute a larger-gap magnet for a poor mechanical door alignment.
Surface, recessed and overhead mounting
Surface contacts are accessible for testing, often convenient on retrofits and easier to position without major structural work. Recessed contacts can look cleaner and provide some protection, but drilling through narrow frames, weather seals or fire-rated assemblies can create larger problems. Use the manufacturer-specified hole size and orientation, maintain sufficient edge distance, and avoid crushing the housing during installation.
On steel frames, use devices qualified for steel or specified spacers where required. A product described as suitable for steel doors should still be checked for the particular installation geometry. A high-mounted contact may avoid accidental damage but can be difficult to service. Route wiring in protected paths with suitable strain relief and access for future fault-finding. Never compromise a certified door assembly merely to hide a detector.
Roller doors need a purpose-selected contact
A roller shutter rarely behaves like a hinged internal door. Its curtain may sway, the bottom rail can vary in height and metalwork can attract impact and vibration. A heavy-duty roller-door contact is usually designed for a physically robust mounting arrangement and an appropriate gap, often at the bottom rail and floor or a suitable guide position. Verify the chosen product's environmental and mechanical ratings rather than relying on its name alone.
Plan for cleaning equipment, forklifts, pallet movement, water and corrosion. Protect the contact from direct tyre contact and avoid a location where debris builds up. The magnet and switch must align reliably with the shutter fully lowered, including when the curtain settles unevenly. Confirm there is no unsafe interference with the door's guides, obstruction sensors or drive mechanism. Automated door equipment requires additional care and coordination with its maintainer.
Separate position monitoring from lock monitoring
A door contact can support forced-open or door-held-open logic, but it cannot independently show that an electric strike keeper has returned or a maglock armature is properly bonded. If the risk requires assurance of secure locking, select a device with the appropriate proven lock-state monitoring and wire that indication to a distinct input. The controller should distinguish unauthorised opening, prolonged opening and lock-failed-to-secure events.
When the door opens after a valid credential, the controller typically suppresses a forced-door alarm during a configured access window. That is not a reason to increase the door-held timer indefinitely. Adjust timers to actual closer performance and traffic conditions. Review door monitoring and REX guidance for the wider event sequence.
Testing and preventative maintenance
Check switch state with the door fully closed, partly open and clearly open. Repeat the test while gently moving the closed leaf within its normal mechanical play to identify borderline gaps. On a roller shutter, test several complete cycles and inspect the cable route for chafing. Verify the exact event shown in the alarm or access control system, including whether supervision faults are identified separately.
Record physical dimensions, measured operating margins, panel zone identification, contact wiring and final mounting photographs. During maintenance, inspect for loose fixings, door sag, corrosion, cracked magnets and new obstacles. Intermittent contact alarms often arise after door repairs or alignment changes. Recheck the detector after any adjustment to the hinges, closer, strike plate or roller shutter guides.
Choose actual products against site conditions
The MS-424M roller-door contact is an obvious catalogue starting point for shutter discussions. Compare it with a surface contact and a recessed contact for other door geometries. These product links identify relevant device classes, not universal interchangeability. Verify each model's exact gap and permitted mounting material from current technical documentation.
Where two physically different openings feed a single alarm panel, document the selection separately. A narrow timber office door, a steel warehouse personnel door and a large loading-bay shutter should not automatically receive the same reed switch. Good selection prevents nuisance events and improves confidence in every later alarm investigation.
Make and break gaps: allow for real door movement
Some reed contacts close their circuit as the magnet approaches at one distance and open at a different distance as it moves away. This difference, sometimes called hysteresis, matters when a door vibrates near the operating threshold. A quoted maximum gap may not say whether it is a make gap, break gap or a result measured with a specific target magnet. Read the current manufacturer's drawing and reproduce its stated orientation during installation rather than assuming that the same distance applies on every axis.
Measure the contact spacing when the door is fully secured, not just when it first touches the frame. Check the largest and smallest spacing expected under ordinary play, wind and temperature changes. The installation should remain consistently in its intended closed state throughout that movement and change state promptly as the door opens. If the operating margin is inadequate, correct the mounting position or door mechanics before commissioning. Do not rely on an unverified numerical gap as a universal safety allowance.
Practical product selection: surface, recessed or roller-door contact?
For a conventional personnel door, a surface-mounted product such as the MS-26ST may make alignment and future service convenient. Where the doorset permits drilling and the required dimensions can be maintained, the MS-11R illustrates a recessed approach. For a moving industrial shutter, the MS-424M is a relevant roller-door product to assess. These are application comparisons only: check their current datasheets for actual switching gaps, wiring configurations, permitted mounting surfaces, mechanical protection and environmental suitability before specifying a model.
On steel, magnet performance can change if the surrounding metal redirects the field. Manufacturer-approved non-ferrous spacers or installation methods may be required. On recessed installations, the door's certification and frame design take priority over concealing a contact. On roller shutters, protected cable routing and a repeatable end-of-travel position are often more important than a larger advertised gap.
Door-contact commissioning checklist for NZ installers
- Confirm the selected contact and magnet are the matched parts, with the correct mounting orientation and current technical specification.
- Inspect hinges, shutter guides, bottom rails, seals and frame movement before fixing the devices in their final positions.
- Measure and record the installed separation and check switching with a meter in both open and closed positions.
- Cycle hinged doors or roller shutters repeatedly, checking vibration, normal play and any change in final resting position.
- Check cable protection, strain relief, fixings and environmental exposure without compromising safe operation or certified assemblies.
- Prove the controller or alarm input displays the expected open and closed state and reports any configured supervision fault distinctly.
- Confirm door-held-open and forced-opening behaviour using authorised and unauthorised test sequences where those functions are configured.
- Document the installed location, photographs, test results, applicable product data and any future maintenance access limitations.
Where motorised doors or safety devices are involved, coordinate testing with the authorised door maintainer and avoid interfering with safety edges, obstacle detection or the drive system. This checklist supports commissioning but does not replace manufacturers' instructions or the applicable approved building design.
Frequently asked questions
Is a larger advertised gap always better?
No. The contact still needs a repeatable state transition with an adequate operating margin, correct magnet orientation and compatibility with the mounting material.
Can a normal door reed be used on a roller shutter?
Use a contact explicitly suited to the shutter's movement, mechanical exposure and required operating gap, with an installation method approved by its manufacturer.
Will a contact prove the door is locked?
No. It indicates the leaf position. Lock mechanism or bond monitoring must be assessed separately.
Why does a contact fail intermittently on steel?
Misalignment, marginal spacing, interaction with surrounding steel, loose mounts and changes in door geometry are common causes. Measure and test the installed assembly.
Explore the access control collection for related product categories and discuss exact device selection with our technical team.
Talk to Essential Security Supplies
For project-specific product selection, current technical documentation and compatible access control or alarm components, contact Essential Security Supplies for specification support or a trade quotation. Share door type, system model, required interfaces and installation environment so the advice can address the actual site. Contact our team. Also see our single-door access control planning checklist for related design questions.
