What do the equipment protection levels Ga, Gb, Gc; Da, Db, Dc mean in explosion-proof electrical equipment?

What do the equipment protection levels Ga, Gb, Gc; Da, Db, Dc mean in explosion-proof electrical equipment, such as eye wash?
Ga: Equipment for explosive gas atmospheres with a "very high" level of protection which is not a source of ignition during normal operation, expected failures or rare failures.
Gb: Equipment for explosive gas atmospheres with a "high" level of protection which is not a source of ignition under normal operating or anticipated fault conditions.
Gc: Equipment for explosive gas atmospheres, with a "general" level of protection, which is not a source of ignition in normal operation, 
and which may take some additional protective measures to ensure that it will not occur in situations where ignition sources are expected to 
occur frequently (e.g. failure of luminaires) form an effective ignition.
Da: Equipment for explosive dust atmospheres with a "very high" level of protection which is not a source of ignition during normal operation, expected failures or rare failures.
Db: Equipment for explosive dust atmospheres, with a "high" level of protection, which is not a source of ignition under normal operating or anticipated fault conditions.
Dc: Equipment for explosive dust atmospheres, with a "normal" level of protection, which is not a source of ignition in normal operation, 
and which may take some additional protective measures to ensure that it will not occur in situations where ignition sources are expected to 
occur frequently (e.g. failure of luminaires) form an effective ignition.

Marst Safety Equipment (Tianjin) Co.,Ltd

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Tianjin, China

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