Short answer: fit a fail-secure electric strike on any enclosure nobody enters — it stays locked through an outage and draws no standing current. Fit a fail-safe strike, or a fail-secure strike with a mechanical inside thumbturn, on any door a person walks out of. NFPA 101 requires egress doors in occupied buildings to open readily from the inside without a key, and electrified locks on those doors to release the moment power is lost. The costly mistake is an exit that depends on electronics: a controller crash or a dead supply locks the technician in with the panel.
What each lock does when the power drops
Fail-secure means the lock needs power to open and nothing to stay shut. A spring holds the latch pointed; a short pulse retracts it, and it relocks the instant the pulse ends. Hardware catalogs call this power-to-open. Between unlocks the strike sits at 0 A, so it adds no heat inside a sealed box, and an outage leaves the door locked.
Fail-safe means the lock needs power to stay shut, which catalogs call power-to-lock. A solenoid draws current for every hour the door stays locked, and cutting the supply springs the latch open. A typical fail-safe strike draws 150-300 mA at 12 V DC, which is about 2-4 W of standing heat. Electromagnetic locks are always fail-safe: a 600 lb [2.7 kN] maglock has no mechanical latch at all, and its 300-500 mA coil releases the instant the supply drops.
That asymmetry settles most of the decision. The secure-by-default lock sips power; the safe-by-default lock spends it around the clock.
Fail safe vs fail secure at a glance
| Property | Fail safe | Fail secure |
|---|---|---|
| State with power off | Unlocked | Locked |
| Current draw | 150-300 mA continuously (about 2-4 W) | A short pulse while unlocking; 0 A at rest |
| Battery backup | Needed to keep the door locked in an outage | Not needed for security |
| Controller crash | Door opens | Door stays locked |
| Egress duty | The standard choice on exit doors | Only with a mechanical free-exit device |
| Typical hardware | Fail-safe strikes, magnetic locks (600 lb / 2.7 kN) | Fail-secure strikes, solenoid bolts, electric cam locks |
| Best fit | Rooms a person enters | Cabinets nobody enters |
Where fail safe wins
Any door a person exits through belongs to fail safe. NFPA 101 requires door leaves to be opened readily from the egress side whenever the building is occupied, with locks and latches that work without a key. For electrified locks the code goes further: the lock must release immediately on loss of power, and sprinkler operation must release it too. A maglock or a fail-safe strike does that by physics, and it is also the fastest exit there is — no button, no wait.
The price is the battery. A fail-safe door is only as secure as its backup supply, because the lock sits unlocked for the whole time the power is down. The worked example below sizes one for a 250 mA strike and a full day of outage. The other half of the design is the inside release: mechanical, keyless, one motion, reachable without climbing over equipment. The full hardware list around a control enclosure is in how to specify secure control enclosure access.
Where fail secure wins
Cabinets and enclosures nobody enters take fail-secure without a second thought. Nobody is inside to trap, so egress never enters the question. An outage or a crashed reader keeps the door locked instead of swinging it open for anyone walking past, and the strike draws nothing between unlocks, which matters in a small sealed box. When the card reader dies, a mechanical key override in the same cylinder gets a technician in for the price of a key.
That is the spec for unattended outdoor electronics — pole-mount cabinets, rooftop radios, remote telemetry. Lockable weatherproof enclosures for unattended outdoor electronics covers the rest of that build, and power feedthroughs for sealed enclosures shows how to bring the strike supply into a rated box without wrecking the seal. Our 304 and 316L stainless boxes for this duty are rated NEMA 4X / IP66 by the manufacturer; not UL listed — what those ratings actually test is in the NEMA 4X vs IP66 guide.
Battery hold-up for a fail-safe strike drawing 250 mA at 12 V
Inputs. Continuous draw of 250 mA at 12 V DC — a typical mid-range strike datasheet figure; replace it with your strike's rated current before you buy. Capacity of 7 Ah from an AGM battery of that voltage, the size most access-control panels ship with. The load runs every hour, because a fail-safe lock spends current keeping the door locked.
The rule. Hold-up time in hours equals battery amp-hours divided by the continuous draw in amps.
The arithmetic. 7 Ah ÷ 0.25 A = 28 h. A magnetic lock draws more: 7 Ah ÷ 0.5 A = 14 h at 500 mA. A large fail-safe strike at 0.9 A drops the same battery to 7 Ah ÷ 0.9 A ≈ 7.8 h.
The decision. One such battery carries a 250 mA strike through a full day of outage, so a day-long utility failure still leaves the room locked. Above about 0.9 A of standing draw, hold-up falls under 8 h; add a second battery in parallel for 14 Ah and about 15.6 h at 0.9 A, or step up a battery size. Keep the battery out of the sealed enclosure. AGM cells can release hydrogen and need a ventilated battery compartment, and a single vent hole drops the whole assembly to that vent's rating.
The decision by environment
| Where the lock lives | Choose | Power and backup | Why |
|---|---|---|---|
| Unattended outdoor or pole-mount cabinet | Fail-secure strike or solenoid bolt | Pulse only on unlock | Nobody inside; an outage keeps it locked |
| Wall-mount enclosure, contents only | Fail-secure strike with key override | 0 A at rest | Service access survives a dead reader |
| Walk-in rack room, occupied building | Fail-safe strike on battery, or fail-secure with a mechanical inside thumbturn | Sized battery carries the 250 mA strike for 28 h | NFPA 101: exit without a key or power |
| Sprinkler-protected room | Fail-safe lock or a release-on-waterflow arrangement | Releases on power loss or sprinkler flow | The code treats sprinkler flow as a release signal |
One person inside the room changes the whole answer: the exit goes mechanical first, and the electronics only ever add convenience on the outside handle.
What to put on the order
Pick the lock function from the table above, then spec the details that keep it working:
- A strike and power supply evaluated to UL 294, the access-control standard, running on 12 or 24 V DC.
- Inside release that is mechanical and keyless — a thumbturn or panic device, one motion, nothing powered in the path.
- The battery in its own ventilated compartment outside the sealed enclosure, charged per the battery maker's instructions.
- Reader, exit-button and HMI cutouts factory-cut in the door so the gasket runs unbroken: mounting an operator panel in a cabinet door without losing the rating shows the layout, and the cutouts, glands and HMI windows guide covers the preparation.
- A key override cylinder on every fail-secure door, so a dead reader is an inconvenience rather than a shutdown.
- Locking for the screen itself: a lockable HMI cover on a NEMA 4X door protects the operator interface while keeping the rating.
Isolate the supply and verify absence of voltage before you wire the strike. Then the box itself: we build the stainless enclosure to your drawing — one piece, every cutout as drawn, delivered in 10 business days from your approval of the drawing — and the 3D enclosure builder turns this lock plan into a cutout drawing. For the material call on washdown or salt-air sites, custom stainless enclosures lays out 304 against 316L.
Catalog dimensions to compare
Compare these catalog dimensions with your component layout. Open each enclosure for its rating, material and usable space, then check those specifications against your application.
| Enclosure | Material | Outside H x W x D / display | Rating |
|---|---|---|---|
| 12 × 10 × 6 in | 304 stainless | 12 x 10 x 6 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 20 × 16 × 8 in | 304 stainless | 20 x 16 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
| 24 × 20 × 8 in | 304 stainless | 24 x 20 x 8 in | Rated NEMA 4X / IP66 by the manufacturer; not UL listed |
Questions
Does a fail-safe strike stay unlocked during a power outage?
Yes — power off means unlocked, which is the point on an egress door. A typical strike draws 150-300 mA, so with no battery the room sits open for the whole outage. Size a backup battery as capacity divided by draw: the worked example below shows the arithmetic for a 250 mA strike. Add a low-battery supervisor so a failing charger announces itself instead of leaving the door open overnight.
Can a fail-secure strike still satisfy egress rules?
Yes, when the inside hardware is mechanical: a thumbturn or lever that retracts the latch in one motion, with no key and no power. NFPA 101 asks that doors in occupied buildings open readily from the egress side without a key; a free mechanical exit meets that even though the outside handle stays locked through an outage. Fail safe still wins where the exit must also release on sprinkler flow.
How long will a 12 V 7 Ah battery hold a fail-safe lock?
Divide capacity by draw: 7 Ah ÷ 0.25 A = 28 h for a typical strike, and 7 Ah ÷ 0.5 A = 14 h for a 500 mA magnetic lock. Two of those batteries in parallel double both figures to 56 h and 28 h. Charge the bank from the access-control supply and keep it in a ventilated compartment outside the sealed enclosure.
Is a magnetic lock fail safe or fail secure?
Always fail safe: the 600 lb [2.7 kN] holding force exists only while the coil is powered, typically 300-500 mA. Cut the supply — or lose it in an outage — and the door releases, with no mechanical latch to fall back on. That makes a maglock the cleanest egress lock and the worst choice for an unattended cabinet unless a battery keeps it powered.
What lock goes on an unattended outdoor enclosure?
Fail-secure: a strike or solenoid bolt that stays locked through outages and draws current only for the seconds it takes to unlock. Pair it with a mechanical key override so a dead reader never strands a service visit, and order the reader and exit-button cutouts factory-cut so the door gasket runs unbroken and the manufacturer-rated NEMA 4X / IP66 seal survives.
