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This is a design that my colleague was using to supply power to some op-amps in an accompanying AC voltage measurement circuit.

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I'm wondering why the ground of the secondary side of the transformer (GNDA) is connected the AC voltage input.

Doesn't this beat the whole point of using the transformer for safe galvanic isolation?

The measuring circuits themselves have isolation ICs in them though.

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The mains transformer input MUST NOT be connected to GNDA.

The mains transformer current should circulate between live and neutral.

The mains side safety ground is just that, for safety. It should not be used for normally carrying current. It carries current if the live to ground insulation is breached in the transformer, and blows a fuse, pops a breaker, or operates a GFCI device.

Once the wiring of the mains input side has been sorted out, then the negative rail of the DC side may be connected to mains safety ground or not, at your discretion.

A larger safety concern is whether the mains transformer actually is connected to neutral, and it's been mis-labelled ground, on the basis that neutral is '0 volts anyway!' Usually, the neutral conductor is 0 V, when the mains distribution is wired correctly, and supplied through a correctly wired polarised plug. Mis-wirings are not uncommon, and if the supply comes through a non-polarised 2-pin plug, which are common in every country in the world, then it's a 50-50 chance whether your 'neutral' wire is roughly ground, or live.

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  • \$\begingroup\$ Non-polarized plugs are not common in all countries, just many. Australia/NZ uses polarized 2-pin plugs. \$\endgroup\$
    – user253751
    Jun 13 at 12:00
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Assuming "GNDA" symbol on the left of the transformer means mains neutral is connected to the circuit ground "GNDA"...

If the purpose of the circuit is to measure mains voltage, and it uses a resistor divider to do so, then it can't be isolated from mains. In this case the circuit makes sense. Of course the whole circuit needs to be isolated from any contact with fingers or anything else, which can be a problem if it has a keyboard, or a microcontroller programming port.

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I’m not sure why your colleague is using ground as a return for AC. If this is from the AC mains then it should certainly return via a neutral conductor - this will eventually be earthed somewhere, but using ‘earth return’ like this is very much frowned upon.

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