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Author Topic: Isolation Transformer Recommendation  (Read 1415 times)
AC2EU
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« Reply #15 on: July 24, 2019, 04:04:23 PM »

Something to be aware of....

Many new "isolation" transformers bridge the safety ground from input to output due to regulations and liability issues.  You have to cut this link yourself to provide true isolation of the ground, as well as the hot and neutral lines.

Brian - K6BRN

When do you need to isolate the safety ground?

any time that the device under test has a chassis potential that is not ground. That applies to ac/dc and some military stuff that runs on battery.
When in doubt use isolation else things go poof or bang.
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KX4QP
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« Reply #16 on: July 24, 2019, 04:16:20 PM »

It's true that if you get yourself across the secondary conductors of the isolation transformer you're likely to be in bad shape, give or take activation of circuit breakers etc. on that circuit.  But the idea is that you are protected from the most common fault situation in which you, yourself, are between an active conductor and ground.  Since the transformer secondary circuit is isolated from ground, your body does not complete the circuit as it would if you touched an active conductor on the primary (mains) side. There are various other reasons for using an isolation transformer but that's the basis of the safety motivation.

I'm not completely sure I follow this.  The "neutral" on the output side of the 1:1 transformer has the same potential to ground as the "neutral" on the input side, does it not?

Regardless, given I work (in my shack) on a plastic table, sitting on a plastic chair on a carpet (over engineered lumber sub-floor), the only ground danger I have is the case of the rig itself -- and the isolation transformer won't help me there, because that case "ground" is the same as the transformer output "ground" -- or it's ungrounded, which means there's no protection from, say, worn insulation energizing the rig's case (though I still don't have an external ground accessible anywhere within reach of the work space).
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AA4PB
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« Reply #17 on: July 24, 2019, 05:10:43 PM »

With an old AC/DC type radio, the chassis was connected to one side of the power cord. The other side went to the rectifier and the series connected tube filaments. There was no transformer. The plugs had the same sized prongs so it could be inserted into the outlet in either direction. Plug it in one direction and the metal chassis was connected to the hot side of the AC line. Plug it in the other way and the chassis was connected to the neutral side of the AC line. It didn't matter so much for the user because the metal chassis was installed in an insulated case and they had plastic knobs on the control shafts. For the service tech who had the radio out of the case, a hot chassis was not very healthy. It's especially bad if you tied to connect a grounded scope probe to the hot chassis! Shops used an isolation transformer to power these radios so that the chassis was never connected to the hot side of the AC line, regardless of which way the plug was inserted.
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Bob  AA4PB
Garrisonville, VA
VK6HP
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« Reply #18 on: July 24, 2019, 05:50:59 PM »

I'm not completely sure I follow this.  The "neutral" on the output side of the 1:1 transformer has the same potential to ground as the "neutral" on the input side, does it not?

No, it does not.  Unless you connect the secondary circuit to ground, it is "floating" and you'll be unable to measure any potential difference between either secondary conductor and ground with a VOM.  There are some second-order effects to do with leakage and capacitive coupling but let's not complicate the safety discussion.

In the workshop situation you're describing, perhaps referencing something like your SB-102 with a "proper" double-wound power transformer, the safest thing to do is to always ensure that the chassis is connected to your mains safety ground.  With a transformerless radio, the safest thing is to use an isolation transformer.  Don't ever rely on the incidental insulation you may get from footwear and floor coverings, unless you're working in a properly equipped HV lab with associated protocols.

73, Peter.
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KX4QP
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« Reply #19 on: July 29, 2019, 04:17:20 PM »

How are any of us over the age of 50 alive?

I remember when I first encountered a polarized plug -- because I couldn't plug it in at all, in the unpolarized, ungrounded outlets in the house we lived in around 1972.  We had a couple grounded appliances (refrigerator, as I recall, and a space heater -- in the bathroom) that were plugged in with those orange ground adapters that connected the ground to the screw holding the face plate on the outlet -- except these outlets didn't even have boxes, never mind grounded boxes.

Does "unable to measure any potential difference" mean the neutral is at ground potential on both sides, or the voltage between them is "undefined" like what you get when you divide by zero?  Or does it mean a meter with a needle will read zero, and a digital one will jump all over the place?

Grounding the chassis of the SB-102 doesn't give me a warm, confident feeling when I'm about to try to poke a short meter probe into the chassis to reach a line expected to carry 800 V.   Shocked
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VK6HP
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« Reply #20 on: July 29, 2019, 06:07:54 PM »


Does "unable to measure any potential difference" mean the neutral is at ground potential on both sides...


It means that if you were dumb enough to touch either one of the isolation transformer secondary conductors while standing on a grounded surface there'd be no current through your body.  (Again: very important not to ground either side of the isolation transformer secondary if you want to use it in this safety role).

The reason I put it in terms of a VOM measurement is that those second order effects I mentioned can be measured with the right equipment, but I'm not going to dilute the essential safety message.

Re the 800 V.  You're a responsible adult who understands the need for caution, are you not?  The 800 V connections are not chasing you around the room; you know where they are and you're going to do it right, right?

Grounding the SB-102 chassis means that if you do accidentally get between active mains conductors and the chassis, your house ground fault devices (such as the residual current detectors we are obliged to have in Australia) activate.  I can't answer for the vagaries of earlier North American mains distribution but it's always a good idea to verify what you're actually working with at a given property, wiring code notwithstanding.

« Last Edit: July 29, 2019, 06:24:08 PM by VK6HP » Logged
AA4PB
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« Reply #21 on: July 29, 2019, 06:15:25 PM »

Grounding the chassis of the SB-102 doesn't give me a warm, confident feeling when I'm about to try to poke a short meter probe into the chassis to reach a line expected to carry 800 V.   Shocked

The other side of that 800V supply is connected to the chassis so if you get between the 800V line and chassis you are going to feel it whether the chassis is grounded to the mains supply ground or not. At least if the chassis is grounded then you know that you won't get a shock if you touch the chassis while some other part of your body is touching some other grounded object.
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Bob  AA4PB
Garrisonville, VA
KX4QP
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« Reply #22 on: July 30, 2019, 04:25:03 PM »

Grounding the SB-102 chassis means that if you do accidentally get between active mains conductors and the chassis, your house ground fault devices (such as the residual current detectors we are obliged to have in Australia) activate.

If you live in a house that wasn't built recently, GFCI outlets or breakers are where you put them yourself.  The house I'm in is a mobile home approximately twenty years old, and there's precisely one working GFCI device in the house -- in the outlet above the sink in the bathroom we don't use (just worked out that way -- and the breaker box is chaotic enough that swapping the protection device or installing more is an unavoidable exercise in restarting and resetting stuff, not to mention costly).  The grounds are good, but as OSHA says, "a ground won't protect you."  In the case of grounding the chassis of a rig like my SB-102, a ground may actually cause an accident, because the space is tight enough that it's almost impossible to make a measurement inside without touching the chassis -- and if you also touch a hot conductor, you will get a shock.  None the less, the HP-23A has a grounded cord (and early one, no doubt), which grounds the radio chassis through the power cable.

I've got a pretty good idea how to protect myself -- I repair power tools for a living, in a shop that has GFCI devices only because I personally insisted on them after a "near miss" where an exposed brush wire in a tool under test electrified the entire row of 4 steel work benches.  It's almost impossible to get the boss to actually replace these devices when they fail, too ("just get me the part number and I'll order what you need" -- except he never does; one bench has a failed device that's been out of service for more than a year), so I've gotten quite "enlightened paranoid" about mains voltage.
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VK6HP
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« Reply #23 on: July 30, 2019, 06:11:36 PM »

I'm glad to hear the HK and its grounding arrangements might yet save you from yourself.  

I don't know of anything that will save you from poor technique in terms of measuring the HV within the radio but you really don't have to be contacting the chassis while you make a measurement - if you find yourself doing that, you're doing it wrong, no matter how small the space. For example, if you get some insulated clip adapters for your test leads you can solidly attach the common lead to a chassis point and an HV point prior to switch on. Usually I just use the chassis clip but spending a few minutes finding orientations and positions that work is always worthwhile.

Having just taken a pretty big hit financially extending a switchboard and associated breaker/RCD system I'm familiar with some of the practicalities.  If it's not possible to upgrade the switchboard right now, why not grab a portable RCD for the radio bench?  If you want to progress you clearly need to find ways forward instead of arguing how the status quo blocks you at every turn.

In that vein, this prattling on the internet is not getting my 630 m reflectometer finished so, with the isolation transformer questions answered, I don't propose to add more.

73, Peter.



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KX4QP
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« Reply #24 on: July 31, 2019, 04:26:45 PM »

This is the first I've heard of a GFCI having any chance of protecting me from the HV -- which is DC with filter capacitors the size of a beer can, near enough.
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VK6HP
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« Reply #25 on: July 31, 2019, 05:15:13 PM »

As it's a safety matter, I again note that protecting yourself from the radio HV and other supply rails is your responsibility, per my previous post. In the hands of a responsible individual, and done properly, testing and repairing classic radios is an entirely tractable challenge.

The mains ground fault device protects you in the event of contact with the mains. In the scheme of electrocution statistics that's a worthwhile contribution to safety and, if the shack power circuit doesn't already have an RCD or similar, the suggestion is to consider adding one to the test bench.

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