Tranformers and Earth.

[gaston.rc]

Hi everyone. I have a 220V / 24V transfomer , Is there a problem if i connect one of the wires of the secundary to the earth?.
My doubt is for example if somebody touch the live wire of the secundary would the RCD of my  house installation ( in the primary side of the transformer) trip? I know that 24V is pretty safe, but in the case that i would have a 220/110V tranformer this would be very dangerous.
Thanks.

[Zero999]

Yes, it's possible to connect one side of the secondary to earth.

No, it won't trip the RCD.

RCDs work by sensing the difference in currents between the phase and neutral conductors. There's no way, a fault on the secondary side of a transformer, can trip an RCD connected to the primary side. The exception being an autotransformer, which is irrelevant in this case.

The only way to get RCD protection on the secondary side of the transformer, is to earth one side of the secondary and put another RCD after it.

A 110V transformer with one side of the secondary earthed and no RCD, is arguably more dangerous, than 220V, with no an RCD.

[gaston.rc]

Thanks you. I thought that but i was not pretty sure. Then can i used the earth to flow current?. To clear out a little bit why i asking all this: I need to sensing when two metals are touched, the main metal part (the chasis) is connected to earth, so what i made is isolate the other metal from the chasis (this metal is in a place that can be touch by someone) and connect to the return wire of a relay. So when the two part touch together, close the circuit and excite the relay. Is safe do this? Is there other way? Sorry if i expand to much.

[Zero999]

Thanks you. I thought that but i was not pretty sure. Then can i used the earth to flow current?. To clear out a little bit why i asking all this: I need to sensing when two metals are touched, the main metal part (the chasis) is connected to earth, so what i made is isolate the other metal from the chasis (this metal is in a place that can be touch by someone) and connect to the return wire of a relay. So when the two part touch together, close the circuit and excite the relay. Is safe do this? Is there other way? Sorry if i expand to much.

My previous post contained an error. Please note the correction.

Yes it's possible to do this, but only if the transformer and relay are run at non-hazardous voltages. I'd recommend keeping the voltage below 30VDC or 12VAC to minimise the risk of shock. The relay should also have a reverse parallel diode (for DC operation) or snubber network/bidirectional transient suppressor diode (AC operation) to prevent high voltages being generated, when the relay is switched off.

[rstofer]

Not only is grounding the secondary possible, it is also a very good idea.  The ground prevents a short, internal to the transformer, from raising the output voltage to 120V + 24V.

Control power transformers in motor controls (480V motor, 120V control) are always grounded.

[Zero999]

Not only is grounding the secondary possible, it is also a very good idea.  The ground prevents a short, internal to the transformer, from raising the output voltage to 120V + 24V.

Control power transformers in motor controls (480V motor, 120V control) are always grounded.

Connecting the secondary of a transformer to earth has its downsides too, one being that if a rectifier is connected to the output, an isolation transformer or differential 'scope probe will need to be used to debug any circuit connected to it.

Modern transformers tend to have double/reinforced insulation between the primary and secondary windings and a thermal fuse which will blow, long before it gets hot enough to damage the insulation, so it's not necessary to earth the secondary for protection against insulation failure.