Battery Cabinet: Swollen Cases, Fan Not Running — Which Hazard First?

Correct answer: A) Step back out and get the room ventilated from outside it, keeping ignition sources out of the space until the atmosphere has cleared

Read the symptoms together rather than one at a time. Swollen cases, warmth and crusted vents on a sealed bank say the same thing: these batteries have been venting, which means they have been gassing, which means hydrogen has been entering a room whose fan is not running. Swelling and heat also point at a charging fault or thermal runaway, so the gassing may still be going on right now. The atmosphere is the one hazard here that can injure everyone in the room at once, and it is the only one that gets worse for as long as a person stands in the doorway deciding. Clearing it comes before anything that makes heat, a spark or an arc.

The detail that makes this hard in the field is that you cannot perceive the dangerous part. Hydrogen is colourless and odourless. The sour smell is the acid, the warmth is the symptom, and the swollen cases are what your eye goes to. Every signal you can actually sense points at something other than the gas. Ranking hazards by how strongly you perceive them is the specific habit this question exists to break.

Sealed does not mean gas-tight. A VRLA cell is designed to recombine the gas it produces internally, and a pressure relief valve exists precisely because that recombination fails under overcharge, high temperature or age. A bank that is visibly swollen has been past that point. This is why VRLA rooms are still ventilated rooms, and why "it is a sealed battery" is not an argument for walking in.

A note on the JTA's equipment list for this task, which reads as insulated tools, a face guard, chemical goggles, eye wash, gloves and aprons. Aprons and eye wash describe the older flooded bank, where routine work meant opening cells and topping them up with liquid electrolyte, and you should not expect that work on a modern sealed installation. The acid has not gone anywhere, though: a ruptured VRLA case will still put it on your skin, so the splash protection still earns its place. The item on that list which never becomes optional is the insulated tools, because a battery bank is a source that cannot be switched off, and a bare wrench bridging two terminals is a dead short on something that will deliver thousands of amps into it.

The O&M point: a commissioning crew meets this bank the week it was installed, in a room just built to specification with a fan that was just tested. You meet it five years later, when the charger has been drifting, the fan has failed and nobody has opened the door since the last inspection. Your work order says "inspect the battery bank." It does not say "handle an unventilated room containing a bank that may be in thermal runaway," and the ability to stop and re-rank the hazards you actually found against the job you came to do is the part no checklist can supply.

Why the other options are wrong

B) Switch the exhaust fan on at the wall switch inside the room, since restoring ventilation is the priority

The priority is right and the execution is what hurts you. A wall switch makes a small arc every time it is thrown, and this one is inside the space you are trying to clear. That is why ventilation controls and lighting for battery rooms are placed or rated so that they are not ignition sources in the first place, and why knowing where that switch is belongs to your preparation rather than to the moment you open the door. Being right about what matters most does not protect you from how you go about it.

C) Get the two swollen batteries disconnected and carried out, since a swelling battery is the hazard that is actively getting worse

The premise is correct, which is what makes this the tempting answer: a swelling battery really is deteriorating while you watch, and it really will not fix itself. But the conclusion sends you into an unventilated room to wrestle with the hottest object in it. A swollen case is already under pressure, handling one can rupture it, and the disconnection work itself means tools on live terminals in an atmosphere you have not cleared. The hazard being urgent is an argument for controlling the room first, not for skipping that step.

D) Open the bank’s main DC disconnect to remove the electrical hazard before doing anything else in the room

This puts an arc-producing action inside a space that may hold an explosive atmosphere, which is option B's error with considerably more energy behind it. It also misreads what a disconnect does here. Opening it separates the bank from the rest of the system; it does not de-energise the bank, because the cells keep their voltage and every terminal in that room stays live. There is no switch anywhere that makes a battery safe to treat as dead.

References

• NABCEP OMAT JTA v.2026.5 — Domain 1, Task 4.D: "Battery safety (e.g., insulated tools, face guard, chemical goggles, eye wash, gloves, aprons)"
• NABCEP OMAT JTA v.2026.5 — Domain 1, Task 3.A names OSHA and NFPA as the safety regulations in scope. OSHA requires battery charging areas to be ventilated so that explosive gas mixtures cannot accumulate, and requires facilities for quick drenching of the eyes and body wherever injurious corrosives are present. No gas concentration or ventilation rate figures are quoted here; the JTA does not test numeric thresholds.