I am working on an old turntable power supply on my workbench. I have acquired a 1:1 bench isolation transformer to increase safety while doing this. It seems to be a well known "issue" with at least the model of transformer I have that the earth ground pin on the 3-prong isolation (output) side is linked to the transformer's chassis-ground and on to the input side (neutral?) and is thus not itself isolated.

My understanding is that this adds risk where risk should be avoided, and some people proactively snip out that earth ground connection from the output pins inside the transformer (effectively disconnecting the earth-ground on the output side and fully isolating the device that's being worked on). Other, simpler isolation transformers don't include a third earth ground pin on either side.

I believe I understand the circuit-completion rationale for this. But what if the device under test has an earth ground pin? Aren't you defeating the safety ground on the device you're working directly on? Is the idea that that no longer matters since its circuit is isolated anyway? Can anyone speak to whether disconnecting the ground pin is as safe, safer, or less safe, or whether once you've done so, you're intended to use it or ground it in some different way? Thanks.

  • \$\begingroup\$ In a fault condition it makes it equivalent to unisolated. One loose wire hits ground and the other wire goes high or low relative to ground and shocks you. \$\endgroup\$
    – DKNguyen
    Commented Mar 11, 2020 at 16:20
  • 2
    \$\begingroup\$ I think you need to show the wiring diagram and terminal layout for your transformer. \$\endgroup\$
    – Transistor
    Commented Mar 11, 2020 at 16:28
  • \$\begingroup\$ "No datasheet? No sale!" \$\endgroup\$
    – Transistor
    Commented Mar 11, 2020 at 16:37
  • \$\begingroup\$ :) OK, here's the spec sheet and the photo from the mfgr but notice the photo shows earth ground connections that aren't shown on the spec sheet jameco.com/Jameco/Products/ProdDS/2214336.pdf jameco.com/Jameco/Products/ProdDS/2214336Drawing.pdf \$\endgroup\$
    – BZo
    Commented Mar 11, 2020 at 16:38
  • \$\begingroup\$ @Transistor ^^ and also from the photo the earth ground appears to be just connected back into input neutral (along with the transformer chassis ground), not input earth ground. I haven't dissected my unit yet, but maybe worth doing if anyone tells me what I should be looking for/doing to increase safety. Thanks \$\endgroup\$
    – BZo
    Commented Mar 11, 2020 at 16:40

1 Answer 1


Having the earth pin of the secondary plugs lessens the safety of the isolation transformer.

The purpose of an isolation transformer is to float the secondary remove reference to earth. What this does, is that if any one connection is made to earth that connection will reference to earth and there will be no difference in potential. For example, if a person touches any one wire and something earthed. The difference in potential will be zero and the person would not get shocked.

The earth pin connected to the chassis of the transformer which is in tern earthed degrades the safety. This is because if one of the secondary phases shorts to the earth pin (chassis), the one wire protection is lost. It is only safe to touch the phase that is shorted. If a person touches the other phase and something earthed they will get shocked.

The drawback of the isolation transformer and not having the cassis earthed is that a fault to the chassis (earth pin) will go unnoticed. If a person touches the none shorted phase and the cassis of the unit they will get shocked. The workaround is to high pot or at the bare minimum to resistance check the earth pin to the other two prongs before working on the device.

  • \$\begingroup\$ Thanks. Can you elaborate on "The workaround is to high pot or at the bare minimum to resistance check the earth pin to the other two prongs before working on the device."? What does it mean "to high pot"? Or in what way wire the earth pin? I'm not a pro electrician, just finding my way. Thanks! \$\endgroup\$
    – BZo
    Commented Mar 12, 2020 at 0:08
  • \$\begingroup\$ @BenZotto There are regulations for how much isolation there must be between the earth pin and the other two. This may be as high as 1000V. A high pot tester applies a high voltage across two pins and measures the current. If the current is greater than a predefined threshold then the device fails. This test finds even low grade shorts to the earth pin. \$\endgroup\$
    – vini_i
    Commented Mar 12, 2020 at 12:48

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.