AC transformer earth connection

[aliciatag]

I am helping out with the installation of an alarm system and need some info regarding earthing of low voltage transformer lead.
The alarm system has a 3 wire 12 volt AC output power supply as well as a back up 12 volt DC battery which also contributes to the AC supply to the main ciricuit board when the system is under load ( When the alarm is sounding).
The system comes with an Earth wire which needs to be attached on one end to the metal housing cabinet and the other end the instructions says should be grounded to the building electrical installation.
My question is: Can I do this grounding by simply connecting the other end of the grounding wire to the Earth wire of the 12 volt AC power supply?

Thanks

Alicia T

 

[vk6kro]

Yes, but you need to check that there is a direct connection between the mains Earth on the power plug and the Earth on the outlet of the power supply.

It seems possible that the battery is there so that the alarm would still work if a burglar turned off the AC power.

 

[Studiot]

My question is: Can I do this grounding by simply connecting the other end of the grounding wire to the Earth wire of the 12 volt AC power supply?

No that is insufficient.

The metal cabinet is a (semi) permanent building installation and as such needs to be permanently bonded (grounded) to the building equipotential grounding system. This will be independent of the presence or absence of the 12 volt supply or its grounding, if any.

The purpose of the grounding is not only to protect against faults in the 12 volts supply but to protect against any fault currents that may occur if neighbouring electrical supplies/apparatus malfunctions.