# Were do ground pins on isolation transformer go [closed]

The outputs from transformer have a ground.

Is output ground earth ground?

Isn't the whole point for those transformers not to provide earth ground potential?

Example:

• "Is output ground earth ground?" You tell us, what is in your device... – Ale..chenski May 18 '18 at 22:25
• I'd pop the case open on that thing and find out, or probe around with a meter – Voltage Spike May 21 '18 at 18:22

No. The point of the transformers is to isolate the live and neutral from ground.

simulate this circuit – Schematic created using CircuitLab

Figure 1. A simplified schematic to show the typical setup.

• At some point in the power distribution one of the supply wires will be "neutraiised" by connecting it to earth. This wire is labelled "neutral" and the other is now live. (This may be done at the local supply transformer or at the supply entry to the building, depending on local regulations.)
• LAMP1 has full mains voltage at its top terminal and, if all is well, 0 V at its bottom terminal.
• XFMR1 isolates the output from the incoming supply. The symbol shows clearly that there is no direct connection between the input and output. We could earth either of the output wires or neither and the circuit will still work.
• In the configuration shown neither L1 or L2 are earthed so the supply is floating.
• Output earth is provided to ensure earthing of any equipment plugged into the transformer. This is to protect the user in the event of multiple faults occurring simultaneously and making the case of the downstream equipment live.

The outputs from transformer have a ground.

Correct.

Is output ground earth ground?

Yes. It is connected to the earth on the input lead.

Isn't the whole point for those transformers not to provide earth ground potential?

It is to remove the earth ground potential or tie on the neutral wire.

• Huh. That's the exact opposite of what we do here in NZ - the earth connection on the output must be disconnected, or in the case of a transformer powering multiple appliances, bonded together but not connected to the mains earth. This prevents the user being placed in an earthed situation. – SomeoneSomewhereSupportsMonica May 19 '18 at 2:26
• That seems unlikely for the type of isolation we're discussing here. Can you point to any regulations so I can read up on it? – Transistor May 19 '18 at 8:07
• ASNZS3000, sections 7.4.5 and 7.4.6. For a plug-in device, ASNZS 3760 Appendix F3 would apply. NOTE – A method is shown in Figure F2. The earth slot in the output socket-outlet must not be connected to accessible earthed parts or the earth conductor in the supply cord or appliance inlet. ` – SomeoneSomewhereSupportsMonica May 19 '18 at 8:17
• ASNZS 3000 7.4.7.2 and 7.4.7.3 would also apply, and requires an insulation resistance test between earth and the earth contacts of the sockets. – SomeoneSomewhereSupportsMonica May 19 '18 at 8:33
• I can't find a copy that's not behind a paywall. I'm located diametrically opposite you in Ireland. – Transistor May 19 '18 at 8:34

It typically is connected to the earth pin on the power cord, for safety, as well as an internal shield between the winding if it's a well-made isolation transformer.

You can easily check it with your ohmmeter, just unplug it and check for continuity between the earth socket and the mains earth pin.

The purpose is not to eliminate the safety ground, it's to have both mains pins floating with respect to that ground. If you are working with an old-fashioned hot-chassis device such, as an "AA5" radio, the cord will have only two pins anyhow.

Personally, I prefer a well-made Hammond product to the offshore branded ones for this kind of application.

Edit: Here is the internal circuit of the isolation transformer series that I use in our lab:

Here is a typical application, debugging a phase controller where the dashed box contains some circuitry and the triac MT1 is connected to the mains "HOT". If the cord to the DUT has a ground pin it will be connected to earth, so in case the triac shorts to the grounded heatsink/case, say, the current will be conducted to earth and the case will remain at a safe potential (and a breaker or fuse will open).

simulate this circuit – Schematic created using CircuitLab

• So no ground loop "explosion" can happen even if isolation transformer provides an earth ground potential to a scope? – user33915 May 19 '18 at 0:23
• @user9762541, wrong!!!!! if the scope probe ground is connected to the scope frame ground and the scope frame is grounded to AC power ground .... you have to use a two prong power cord for the scope when connecting to an isolation transformer .... the scope ground cannot be connected to the mains ground – jsotola May 19 '18 at 1:11
• Yes, it can be, and it should be, for safety. You just cannot have two connections that complete a circuit. The isolation transformer floats the mains so you can ground either side to earth. Everything you are measuring ('mains' powered) is powered through the isolation transformer. Floating the oscilloscope above ground is a very bad practice, equivalent in most cases to just breaking the earth pin off the power cord of the oscilloscope. Quite dangerous. – Spehro Pefhany May 19 '18 at 1:13
• If the circuit is powered through the isolation transformer then connecting either side of the isolation transformer secondary to earth (directly or through the scope probe ground lead) should not cause problems. You're supposed to be isolating the circuit, not floating the oscilloscope, right? – Spehro Pefhany May 19 '18 at 1:19
• @SpehroPefhany Correct. Don't float the scope, or all the metal connections on it will become live. Floating your circuit allows you to connect any one part to earth without damage. – SomeoneSomewhereSupportsMonica May 19 '18 at 8:26