General > General Technical Chat
Isolation transformer - Voltage between output and ground
Ian.M:
OTOH Andy's live heatsink is now grounded. Its questionable whether this is safer than a heatsink that is live with half-wave rectified mains (and note that many older RCDs don't meet their AC trip current specs in the presence of DC leakage current due to core saturation), as the DC bus will have 325V or more on it, and is capable of delivering plenty of current, and has a fair bit of stored energy, so could easily be lethal.
Anyone who's ex-service trade, particularly CRT TVs, will be familiar with this setup, and back in the day, would have insisted on the rubber floor mat as S. Petrukhin suggests, + removing or insulating all unnecessary grounds from the bench area (apart from the CRO which had to be grounded), and even then would treat the circuit board with extreme caution.
Nowadays, there's little excuse, as a Cat II rated differential probe is certainly cheaper than the pine box it will keep you out of!
bdunham7:
--- Quote from: Siwastaja on February 11, 2024, 05:54:45 pm ---Note to those who don't understand why this is a troll: isolation transformer here gives you the false sense of security, while the oscilloscope ground lead re-establishes the galvanic connection to building earth, therefore making the primary side, which andy2000 does not worry about touching, lethally dangerous again.
--- End quote ---
I don't think andy2000 is a troll nor is he wrong. There are legitimate methods of using an isolation transformer and accidental touch contact on live chassis was the main reason they were introduced. For some reason this generates a lot of flipped wigs over the false sense of security issue. Mitigating signficant touch voltage hazards (chassis, heat sink, etc) doesn't mean you can poke your sweaty fingers into every nook and cranny and it never has. It doesn't matter if it is an old radio with a 50% chance of a live chassis, a television with a 100% chance of the chassis being live because it is connected to the negative of a bridge rectifier, or a typical SMPS. Those devices all have significant voltage hazards whether isolated or not, although if you avoid grounding anything you do mitigage single-point contact voltage hazards.
For an SMPS, if you use isolation and ground both the negative of filter cap (which is likely to also be the big heat sink) and the case, you mitigate both those potential touch hazards and set things up to easily take measurements of the rest of the primary circuit. Are there now hazardous ground-referenced voltages on the rest of of the primary circuit? Yes, there are. So be careful!
jonpaul:
Rebonjoyr, been a power electronics eng and consultant since 1970s, medical, avaionics, lighting, etc.
Always used 1:1 isolations trsf on 120 and 240 mains, and variacs in the lab from 100 VA to 25 KVA 3 ph.
Staco had metered isolated Variacs for the lab and Signal (now Belfuse) has excellent DU-..DU-5 isolation trasf 1..5 KVA 120/240 dual pro sec.
Ran equip on sec chassis at earth and scope at earth.
With TEK and Pearson CT, current probes and TEK X10 and X1000 voltage probes, never had any accidents or shocks due to the mains.
Just my experience
Jon
Zero999:
--- Quote from: bdunham7 on February 11, 2024, 06:57:36 pm ---
--- Quote from: Siwastaja on February 11, 2024, 05:54:45 pm ---Note to those who don't understand why this is a troll: isolation transformer here gives you the false sense of security, while the oscilloscope ground lead re-establishes the galvanic connection to building earth, therefore making the primary side, which andy2000 does not worry about touching, lethally dangerous again.
--- End quote ---
I don't think andy2000 is a troll nor is he wrong. There are legitimate methods of using an isolation transformer and accidental touch contact on live chassis was the main reason they were introduced. For some reason this generates a lot of flipped wigs over the false sense of security issue. Mitigating signficant touch voltage hazards (chassis, heat sink, etc) doesn't mean you can poke your sweaty fingers into every nook and cranny and it never has. It doesn't matter if it is an old radio with a 50% chance of a live chassis, a television with a 100% chance of the chassis being live because it is connected to the negative of a bridge rectifier, or a typical SMPS. Those devices all have significant voltage hazards whether isolated or not, although if you avoid grounding anything you do mitigage single-point contact voltage hazards.
For an SMPS, if you use isolation and ground both the negative of filter cap (which is likely to also be the big heat sink) and the case, you mitigate both those potential touch hazards and set things up to easily take measurements of the rest of the primary circuit. Are there now hazardous ground-referenced voltages on the rest of of the primary circuit? Yes, there are. So be careful!
--- End quote ---
You could earth it via a Hall effect leakage current sensor, connected to the breaker, to form an RCD on the secondary side, which would trip, if there's any leakage. It would be very handy to integrate such a device into an isolation transformer box.
andy2000:
I didn't mean to imply that using an isolation transformer somehow makes working with a live mains powered circuit free from danger! Safe working practices are as important as ever since there are still lethal voltages involved. It simply turns a line connected chassis into an isolated chassis, so it's the same as if you working on a device with a power transformer.
There's nothing inherently dangerous about a floating chassis. Most consumer electronics sold in the US comes with a non-grounded plug, and therefore has a floating chassis. Isolation is invariably provided by the transformer in the power supply of the device.
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