Positive Pressurize enclosure

[CoralUK]

“According to the safety requirement, the indoor pressure
for the accommodation and operation areas should be kept to a level of 50 Pa above the outside atmosphere
pressure as all the time as possible. This high indoor pressure is used to prevent eventual gas escape or fire to
enter the indoor areas;”

I also know that the indoor pressure need to keep 50 Pa above the outside atmosphere pressure to prevent intrusion.
I have a question that come to my mind, why the value is 50 Pa? Any proof why is 50 Pa? why not 30, 40 or 60 pa?
Can you provide me the information since I cannot dig it out from book and internet? Hope you can help me with useful information.
Thanks.

 

[JLSeagull]

or 100 Pa?

I don't think that the selection of 0.1-inch of water was very scientific; and it is 25 Pa if you are addressing NFPA 496.

NFPA mentions history dating back to 1966 for standard 496. ISA also had a standard for small enclosures. I think that NFPA may have applied to control buildings. Today, NFPA 496 2008 applies to small enclosures, analyzer shelters and buildings.

NFPA requires maintaining the protected enclosure at a positive pressure of at least 25 Pa (0.1 in. of water) above the surrounding atmosphere during operation of the protected equipment. Maintain a pressure of at least 25 Pa (0.1 in. of water) in the control room with all openings closed. Provide a minimum outward velocity of 0.305 m/sec (60 ft/min) through all openings capable of being opened. The velocity shall be measured with all these openings simultaneously open and a drop in pressure below the 25 Pa.

 

[CoralUK]

Thank you JLSeagull.
How can I get the standard for NFPA 496?