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Old 19th May 2012, 10:48 PM   #11
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if you ground the output of an isolation transformer it pretty much defeats the purpose of having one.
99% of the transformers on this planet are isolating types, and 99% of those are in fact grounded on their secondary. The term 'isolating' is generally unnecessary; it is a given. Any type of autotransformer or variac is understood to have a common connection, and those become the exception to the rule. There are two common options for applying to the secondary:

1. Float the entire secondary, which enables flexibility with what is being powered. We can float our oscilloscope to make floating measurements (with great caution) or we can use it to power medical equipment to prevent shocking the patient. Generally, floating systems are to be avoided, code discourages it, and IEEE does not recommend it. They are only designed for a few specific applications. I see DIY'ers floating it as a band-aid to a larger grounding problem, so the claim is that floating is the way to go (since it 'solved' their problem). This is unfortunate.

2. Ground the secondary. The preferred method for non-testing purposes. The purpose of the isolating transformer has STILL performed its function; it has galvanically isolated the primary system from the secondary system, and provided a separately derived system. It will be very effective in eliminating ground loops. Misunderstood in great measure on this forum, unfortunately.

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The Primary or Input side should be grounded.
Do not EVER ground the primary side. The primary system is grounded back at the panel, and that should be the ONLY place it is grounded. Basic code requirement. You can take a third wire and 'bond' the case, shield, and core, but this is altogether different than grounding the primary winding.
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Old 20th May 2012, 12:44 AM   #12
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These are some great responses! Apparently, there are some differences of opinion.
Yes, this transformer has 3 secondary outlets. But since I'll be using an autotransformer along with it, the 125v outlet will be the only one used. I acquired this transformer in 1981 and it looked old at that time. but it doesn't appear like it's ever been opened.
My custom made Variac has a ground wire connected to the transformer shell and the receptacles. If I use an ungrounded isolation transformer before it, then I'll need to cheat the ground pin. Is this correct?
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Old 20th May 2012, 01:05 AM   #13
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Can you be specific in what you intend to do with this? If it's for testing and playing around with recent builds on the bench, then a floating output is fine. If you want to use it in a semi-permanent installation, there should be a ground reference.

Not sure what you mean by cheating the ground pin. Every enclosure and core should be grounded by means of the 3rd prong when it's available. Whether or not the transformer secondaries are bonded is a separate issue, and the one being debated. Maybe this is deviating from your original request, of which I would apologize.
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Old 20th May 2012, 01:40 AM   #14
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I'll be using it for testing and troubleshooting vintage tube radios and audio components, which I've been doing for 5 years without a safety net.
I rekon I've pushed my luck a little long enough.
So pardon me, but I need to clearly understand two things; do I use this transformer upstream or downstream of the variac and do I add a ground connection to its case?
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Old 20th May 2012, 07:23 AM   #15
cihtog is offline cihtog  United States
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You don't need to cheat anything - the 3rd prong ground contributes little to a working circuit, other than than a measure of safety, and where the iso is concerned, a connection to ground for the shielding.

However, what you do need to do, is verify the connections internally and externally of both devices, before joining them together as a bench tool. Doing things without a safety net, and knowing it, is entirely different from not having one, and thinking that you do.

Work out schematics for both devices - then decide on which is better, first or last. It would also probably be a good idea to figure out which wire tap, out of the 3, is the 1:1 tap as well, to simplify things a bit.
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Old 21st May 2012, 01:27 AM   #16
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Originally Posted by A Sound Mind View Post
do I use this transformer upstream or downstream of the variac and do I add a ground connection to its case?
you can install it up or down, but I would suggest upstream to get the optimal xfmr utilization.

Grounding cases and cores is always a good thing
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Old 21st May 2012, 02:20 AM   #17
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Good advice zigzagflux.
Thank you!
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Old 23rd March 2013, 09:27 AM   #18
Jolida is offline Jolida  India
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Originally Posted by jitter View Post
Agreed for testing purposes the output of the isolation transformer should be left floating. But beware, simultaneously touching live (L) and neutral (N) wires at the isolated end of the transformer remains just as dangerous! Only the (more common) mistake of touching L and e.g. case is not dangerous anymore.

Having said that, there are situations in which it is necessary to make an isolated supply act as a normal one. PE and N are interconnected on the secondary side (e.g. on a boat that gets an isolated power supply from shore to prevent electrolytic corrosion, see image). PE on primare side and secondary side are NOT interconnected, though. I didn't make that clear in my first post.

But for testing purposes, only use L and N to power a device, do not connect PE!
Sorry to revive an old thread, but I wanted to clear a doubt. My isolation transformer is connected to the wall outlet (3 prong), then its output is fed to a voltage stabiliser which then feeds to my gear. So the correct way is to just connect the IT to the phase & neutral of the wall outlet & connect the earth only to its chassis? So the stabiliser or components that come after the IT are not connected to Earth even on their chassis? All my power cords are shielded, so is it ok to not ground the shield then???
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