Electric Latch Retraction: Powering the Exit Device Latch

What It Does

An exit device has a latchbolt that a person releases by pressing the push bar or touchpad. ELR adds a powered mechanism inside the device that can do the same thing on command. Von Duprin describes two typical uses:

• Momentary unlatching: the latch is retracted briefly, "often... with a credential reader and access control system to unlock the door momentarily for authorized users."
• Extended retraction: the latch is held retracted "for an extended period of time to allow free entry," which Von Duprin calls "a convenient alternative to mechanical dogging."

Dogging means holding the latch retracted so the door can be pushed or pulled open without latching. Von Duprin also lists hands-free automatic door operators as an application for ELR.

Egress Is Always Mechanical

ELR controls entry and passage. It does not control exit. Von Duprin's QEL data sheet states that "these devices always provide mechanical egress," and a Locksmith Ledger article on the earlier EL design notes that "mechanical egress is always available by pressing the pushbar."

This is the key difference from locks that hold a door closed electrically, such as electromagnetic locks. With ELR, a person leaving can always open the door by pressing the bar.

Solenoid and Motor Designs

Two general mechanisms appear in manufacturer and trade descriptions:

• Solenoid: Locksmith Ledger describes Von Duprin's original EL option this way: "When power is provided, the solenoid retracts, pulling the latching mechanism to the unlocked position, while at the same time, retracting the pushbar."
• Motor: Von Duprin's QEL (Quiet Electric Latch retraction) uses "motorized latch retraction." Von Duprin markets it for hospitals, libraries, museums, and theaters "where ambient noise can be disruptive."

Von Duprin lists several features of its motorized design, including on-board diagnostics, automatic adjustment of "latch throw and pull," and detection of tampering. These are features of one manufacturer's product line, not of ELR in general.

Fail-Secure by Nature

Fail-safe and fail-secure describe what electrified hardware does when power is lost. An NIH technical bulletin explains that electric latch retraction "allows for latch retraction when power is applied; when there is no power, the latch is extended. Electric latch retraction functions are therefore fail secure." The bulletin adds that "the touchpad of the panic hardware provides free egress."

In other words, if power fails, an ELR door goes back to latching like an ordinary exit device. The outside is controlled by whatever trim the device has, and the inside still opens with the push bar.

ELR vs. an Electric Strike

An electric strike releases the door at the frame: the keeper moves so the latch can pass. ELR releases the door at the device: the latchbolt itself is pulled in. Both are ways of letting an exit device door open on an access control signal. Which one suits an opening depends on the device type, the frame, and any fire rating, so the manufacturers' compatibility information governs.

Fire-Rated Openings

Fire doors must latch. Von Duprin's QEL data sheet notes that the option "is available on panic and fire rated devices," but that "fire rated openings have several additional requirements. The opening must be self latching and self closing and under the control of an automatic fire alarm system." The NIH bulletin likewise notes that EL devices can be used on fire doors with a signal from the fire alarm system to the power supply so the door latches during an alarm.

Von Duprin also notes that its hex-key manual dogging option for QEL is ordered "on non fire-rated devices."

Detailed requirements depend on the listing of the hardware, the edition of the codes adopted, and the authority having jurisdiction (AHJ). See fire-rated door hardware.

Power and Wiring at a Glance

ELR devices need a power supply and a path for wiring from the frame into the door. Von Duprin's QEL works with a standard 2-amp power supply, and its data sheet covers wire-run lengths and electric power transfer devices that carry wires across the hinge edge. Exact voltage, current, and wiring are product-specific and belong to the manufacturer's instructions.

Standards and Listings

Von Duprin states that its devices keep their ANSI/BHMA A156.3 Grade 1 certification with QEL fitted, and that "the QEL is UL and cUL listed for use on panic exit hardware and fire exit hardware." ANSI/BHMA A156.3 is the American standard for exit devices; see ANSI/BHMA lock grades for how BHMA grading works.

Related Topics

• Panic hardware: history and function of exit devices
• Electric strikes: releasing the latch at the frame
• Electromagnetic locks: fail-safe holding force
• Access control: credentials and controllers
• Card readers
• Fire-rated door hardware
• Delayed egress locks
• Door hinges
• Removable mullions

Sources