Q.Is there any SOLAS regulation which states that during cargo operation on tanker vessels, we cannot test GMDSS equipment ?

ISGOTT 2.7

During medium and high frequency radio transmission (300 kHz — 30 MHz), significant energy is radiated which can, at distances extending to 500 metres from the transmitting antennae, induce an electrical potential in unearthed ‘receivers’ (derricks, rigging, mast stays etc.) capable of producing an incendive discharge. Transmissions can also cause arcing over the surface of antenna insulators when they have a surface coating of salt, dirt or water. It is therefore recommended that:

All stays, derricks, and fittings should be earthed. Bearings of booms should be treated with a graphite grease to maintain electrical continuity.

Transmissions should not be permitted during periods when there is likely to be a flammable gas in the region of the transmitting antennae.

Low energy transmissions, such as are used for satellite and VHF communications, do not produce the same sources of ignition. 

ISGOTT 4.11.2 

The use of a tanker’s radio equipment during cargo or ballast handling operations is potentially dangerous (see Section 2.7). This does not apply to the use of permanently and correctly installed VHF and UHF equipment, provided the power output is reduced to one watt or less.

The use of VHF/UHF radio equipment as a means of communication should be encouraged whenever possible.