Stored Energy: Pressure and Charge Decision Tree
Why this matters
A machine that is "off" is not always safe. Capacitors hold a charge after the power is cut. Springs stay compressed. Tanks, lines, and hydraulics stay pressurized. Suspended loads stay up only as long as whatever holds them does. Stored energy does not care that the switch is in the off position. It releases the instant you give it a path, and that path is often your hand. This tree puts the safety action first, then the diagnosis, because with stored energy the order is the whole point.
Start here: de-energize and discharge before you diagnose
Do not troubleshoot, test, or open anything until stored energy is removed and proven gone. This comes before any fault-finding.
- Identify every energy source. Electrical, pressure (air, fluid, hydraulic, refrigerant), mechanical (springs, counterweights, suspended loads), thermal, and gravity. Most equipment has more than one.
- Isolate each source. Open and lock out the disconnect. Close and lock the supply valves. Block or restrain mechanical elements that can move.
- Lock and tag it (lockout/tagout). Apply your own lock and tag so no one can re-energize while you work. If multiple people are exposed, each applies their own lock.
- Discharge and relieve. Bleed pressure to zero through the proper bleed point. Discharge capacitors and stored electrical charge using the correct method and rated tools. Release or restrain spring and mechanical energy in a controlled way.
- Verify zero energy. Prove it, do not assume it. Read voltage on a meter you have verified (live-dead-live). Confirm gauges read zero at the work point. Confirm nothing can move. Only after verified-zero do you proceed.
If you cannot positively isolate, discharge, and verify a source, stop and get the means or the help to do it. Do not "work around" live stored energy.
If the energy is electrical charge
Capacitors and similar storage hold a dangerous charge after disconnection, sometimes for a while.
- If the equipment contains capacitors or charge storage: assume they are charged until proven otherwise. After lockout, discharge them through the correct method and confirm with a verified meter that the charge is gone.
- If you cannot confirm the discharge: treat it as live. Do not bridge terminals with your tools or body.
- Only once you read verified zero across the storage device do you handle it.
If the energy is pressure
Pressurized air, fluid, hydraulic, and refrigerant systems store energy that can release violently or spray.
- If the system holds pressure: relieve it to zero through the designed bleed or relief point before loosening any connection. A fitting cracked under pressure can whip, spray, or inject.
- If a gauge still shows pressure, or you cannot see a gauge at the work point: it is not safe. Keep relieving and verify zero at the actual joint you will open.
- Beware trapped pressure between closed valves. Isolated sections can hold pressure even when the main is off. Relieve those sections specifically.
If the energy is mechanical or gravity
Springs, counterweights, raised platforms, and suspended loads store energy in position.
- If something is under spring tension or compression: restrain or release it in a controlled way before you free it. A released spring is a projectile.
- If a load is raised or suspended: support it independently before you work under or near it. Do not rely on the lifting mechanism, a hydraulic hold, or a single point to keep it up.
- If a part can swing, drop, or roll once you remove a restraint: block it first.
Quick reference: source, action, proof
| Stored energy | Make it safe | Prove it is gone |
|---|---|---|
| Electrical supply | Disconnect, lock, tag | Verified-zero voltage (live-dead-live) |
| Capacitive charge | Discharge via correct method | Verified-zero across the device |
| Pressure (air/fluid/refrigerant) | Relieve through bleed point | Gauge zero at the work joint |
| Spring/mechanical | Restrain or controlled release | No tension, cannot move |
| Suspended/raised load | Independent support or block | Load held by something other than the mechanism |
After zero is verified, then diagnose
Only once every source is isolated, discharged, and proven zero do you begin testing and repair. If at any point the job requires energy present to diagnose (some live testing genuinely does), that is a separate, deliberate, properly-controlled live procedure with the correct rated tools and PPE, not a casual continuation. When in doubt, the answer is to stop, re-isolate, and verify again. Stored energy gives no second chance and no warning.
References
- OSHA control of hazardous energy (lockout/tagout) standard, 29 CFR 1910.147
- OSHA electrical safe work practices, including verification on a known source
- NFPA electrical safety in the workplace guidance for de-energized verification and capacitor discharge
- See related: The Instrument You Don't Trust Decision Tree; When to Call for a Second Set of Hands Decision Tree