1. How Pressurized Explosion Protection Works (Pressure > Ambient)
For hazardous gas to enter the cabinet, it must flow against the pressure differential — and a pressurized cabinet always maintains an internalat least 50 Pa above ambient(typically 200–500 Pa) positive pressure, so gas only flows out, never in. By protection level px/py/pz: px reduces Zone 1 to a safe area, py reduces it to Zone 2, and pz is used in Zone 2. The enclosure must have a certain degree of gas tightness (controlled leakage rate), with flow compensation maintaining pressure.
2. Purging (Air Change): The Mandatory Step Before Startup (Interlock Blocks Power-Up)
Combustible gas may have seeped in while the cabinet was shut down, sopurging must be completed before power is applied:
- Feed in ≥ 5 times the cabinet's internal volumeof protective gas (the multiplier specified in the certification documents);
- Purge time = volume × multiplier ÷ flow rate, automatically timed by the system;
- Before purging is complete, the interlock blocks power to the main circuit— this is a mandatory requirement of GB 3836.5;
- Once purging passes, the system automatically raises pressure to operating level, and only then is closing the breaker permitted.
3. Loss-of-Pressure Protection (What Happens When Pressure Drops)
| Pressure Status | System Action | Recovery Method |
|---|---|---|
| Below alarm value | Audible and visual alarm prompts air make-up | Automatically repressurizes once the air supply recovers |
| Below the power-cutoff value (e.g., 50Pa) | Interlock cuts the main power (or a certified time delay applies) | Power cannot be restored until a fresh purge cycle passes |
| Air supply interruption | The exhaust valve closes to slow pressure loss and buy time to respond | After the air supply fault is identified, restore manually |
4. Air Supply and Positive Pressure System Configuration (4-Piece Set)
- Air supply: clean, dry air (instrument air) or inert gas (nitrogen), requiring a pressure reducing valve + filter;
- Flow and pressure monitoring: flow meter, micro differential pressure switch/transmitter; the monitoring point is at the lower section (the deposition zone for gases heavier than air);
- Exhaust outlet: routed to asafe areafor discharge, never toward the operator side;
- Interlock control unit: purge timing, pressure interlock, alarm output, intrinsically safe / flameproof certified.
5. Operation and Maintenance Essentials (3 rules)
① Do not tamper with the pressure relief port: the pressure relief valve orifice is part of the certification and must not be adjusted without authorization; ② Do not bypass interlocks: shorting out the purge or pressure-loss interlock leaves the cabinet unprotected, and any incident becomes your full liability; ③ Log the air supply: check air supply pressure and filter elements monthly, calibrate the micro pressure switch quarterly, and keep records on file.
FAQ FAQ
Why must a positive pressure cabinet be purged before power is applied?
Combustible gas can seep into the cabinet during shutdown; energizing it directly means striking a spark inside an explosive atmosphere.The purge interlockis a mandatory requirement of GB 3836.5.
Can a standard PLC be installed inside a positive pressure cabinet?
Yes— This is exactly the core value of the positive pressure type: the interior is classified as a safe area, so standard industrial components can be used.
Is there a delay before power is cut on pressure loss?
Per the certification settings: generally the alarm comes first and the power cutoff follows; a short delay can be set for critical loads, but the interlock logic is governed by the certificate documents.
What should be noted when nitrogen is used as the protective gas?
Opening the door of a nitrogen-filled cabinet carries an asphyxiation risk, so the operating procedure must requireventilate first, then open the doorLimited-space entry procedures apply as well.
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