By Kent · Lock Depot Staff Writer

Hardwired or battery: the first question on any Detex EAX

Detex sells its EAX door alarms in two families that look alike on a drawing and behave nothing alike on the wall. The EAX-3500 runs on a rechargeable battery pack and gives staff a timed key bypass. The EAX-4200SK runs on building power and ties into the building's alarm panels through relay outputs. Pick by how the door is used and who has to hear the alarm, not by price. If you came here from the battery-powered models, our write-up of the EAX-2500 against the EAX-3500 covers that side in more depth.

EAX-4200SK: the one wired to the building

The EAX-4200SK is a full door management alarm. The catalog lists it as 12/24VDC hardwired, surface mount, with a built-in sounder plus a relay output for tying into building systems. It monitors four things: door status, alarm, bypass and tamper. It is UL Listed, meets NFPA 101, and the listing says it integrates with building fire alarm systems per NFPA 72. It is also listed as ADA compliant.

In the box you get the alarm unit, a magnetic contact, wiring connections, mounting hardware and installation instructions.

The reason to buy it is that nobody has to remember a battery. The listing points to hospitals, detention and government buildings, where a dead battery on an egress door alarm is a finding waiting to happen. The relay outputs are dry contacts for alarm, bypass and tamper status. Those land on a commercial security panel or a building management system, so someone at a desk sees the door go, not just whoever is standing near it.

One thing to plan for: the base unit is hardwired only. There is no battery backup in the unit. The catalog says to use a UPS or the building's emergency power circuit if you need ride-through, and notes the relay output can also trigger on power loss. If a facility wants the alarm alive during an outage, that is a power design question you settle before the order, not after.

EAX-3500: the one that re-arms itself

The EAX-3500 solves a different problem. Staff disable exit alarms for deliveries, smoke breaks and trash runs, then forget to turn them back on. The 3500 uses a timed bypass. An authorized person turns a key, the alarm is bypassed for a set period, and it re-arms on its own when the timer runs out.

• Power: rechargeable NiMH battery pack. The listing says it typically lasts 3 to 5 years and gives a low-battery indicator well ahead of failure.
• Sound level: 100dB.
• Bypass timer: adjustable from 15 seconds to 4 minutes.
• Door type: single doors.
• Finish: gray.
• Listings: UL Listed, meets NFPA 101, ADA compliant.

The bypass needs a physical key turn. Without the key, the unit acts as a standard exit alarm: open the door and it sounds. That matters on a back door where you want staff to have a way through but not a way to leave it silent.

SK or FK on the 3500: how it mounts

The suffix on the 3500 is about mounting. The catalog lists SK as surface mount and FK as flush mount.

• The EAX-3500SK is the surface version, and its listing says it includes a magnetic switch kit for monitoring door position.
• The EAX-3500FK is the flush version. The alarm mounts inside the door header, which the catalog calls a clean, tamper-resistant installation.

Flush means someone has to open up the header, so it suits new construction or a frame that is already being worked on. Surface is the retrofit choice. If the frame is hollow metal and you do not want to cut it, go with the SK.

Side by side

• Power: 4200SK is 12/24VDC hardwired. 3500 is a rechargeable battery.
• Tie to building systems: 4200SK has relay outputs for alarm, bypass and tamper. The 3500 listing does not describe a relay output, so we do not claim one.
• Timed bypass: listed on the 3500, 15 seconds to 4 minutes. The 4200SK listing names a bypass status it monitors but gives no timer range, so check before promising one.
• Mounting: 4200SK is surface. 3500 comes surface (SK) or flush (FK).
• Maintenance: 4200SK has none beyond testing. The 3500 has a battery you will eventually swap or recharge.

What ships with each

The 3500 listing says the box holds the alarm unit, the rechargeable battery pack, a magnetic contact, mounting hardware, keys and installation instructions. The 4200SK box holds the alarm unit, a magnetic contact, wiring connections, mounting hardware and instructions. Notice what the 4200SK does not include: a power supply. The listing says 12/24VDC, so a supply or an existing building circuit at that voltage has to come from somewhere else. Price that in before you compare the two units on the shelf.

Questions to settle before you order

• Is there power at the header? If a 12 or 24VDC circuit is not already near the door, the battery unit may cost less to install even if it costs more to maintain.
• Who has to hear it? A sounder that only the people at the door can hear is an annoyance. A relay output that reaches a security desk is supervision.
• How many legitimate trips per day? A door staff use all day wants the timed bypass. A door that should almost never open wants the hardwired unit and no bypass habit at all.
• Single door? The 3500 is listed for single doors. If you have a pair, tell us and we will check the layout against the listings.

How to choose

Choose the 4200SK when the alarm has to report to a panel, when the facility cannot tolerate a battery going flat, or when an inspector or insurance carrier wants the door supervised. Choose the 3500 when it is a stand-alone door, staff make regular legitimate trips through it, and you want the alarm to put itself back on guard. For a lighter duty door that only needs a prop alarm, the EAX-500 against the EAX-300 comparison is the shorter answer.

One honest limit: the catalog does not give a wiring diagram, relay contact rating or a current draw for the 4200SK. Get those from the Detex installation instructions that ship with the unit before you size a power supply. If you are matching an alarm to a specific exit device and panel, send us the door and the panel model and we will say which of these fits before you order. Browse the full Detex alarm hardware range for the rest of the line.