Well said, Steve!
While power by isolation transformer itself is a very safe thing,
people usually defeat the safety mechanism instantly by producing the
first fault with the scope probe ground.
Isolated power is dangerous in a way, because while it is tolerant
towards the first fault there is no indication this first fault has
occurred and the second fault may be deadly.
This is why isolation monitors exist.....
Many other protection systems (GFI, earthing, ...) show you in some
way that a fault has occurred (usually by clanging noise, followed by
darkness, followed by swearing).
ST
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On Tue, Mar 12, 2013 at 3:31 PM, Steve <ditter2@...> wrote:
--- In TekScopes@..., KeepIt SimpleStupid <keepitsimplestupid@...> wrote:
An isolation transformer allows you to make measurements safely. You don't isolate the line powered instruments, but isolate the load. You don't ever want the case of an instrument to become live.
I don't quite agree with the phrase "An isolation transformer allows you to make measurements safely."
While using an isolation transformer can let you make a floating measurement not referenced to ground, doing so safely requires a keen sense of what you are doing and where the potentials are. A few "newbies" often read this forum for advice, and I would not want them to believe that an isolation transformer magically makes floating measurements safe.
Keeping the measurement instrument (usually a scope) grounded and floating the Device Under Test (DUT) only shifts the place where the user can be exposed to a shock hazard. While it generally will protect the measurement instrument, this practice remains extremely dangerous.
Here is an example. Assume the user is troubleshooting an old Tek scope, as the original poster mentioned. They want to measure a signal referenced the to the +225 V power supply. The usser keeps the test scope safely grounded, so they are not "floating the scope", and plugs the DUT , a 545A scope, into an isolation transformer. The user connects a probe to the test scope, and connects the probe ground lead to the 545A +225 V power rail. There is no spark, and neither the test scope or the DUT are damaged. However, when the 545A is turned on, its chassis, front panel, and all exposed metal parts are now "floating" to ¨C225 Volts! While the user will not get shocked touching the controls on the test scope, which is safely grounded, touching the aluminum front panel on the 545A they are troubleshooting will deliver a painful shock at the least, or possibly a much more serious injury at the worst.
If the user is working on a high power switching power supply, the capacitance of all but the most expensive "medical grade "isolation transformers can cause the supply to explode if the probe ground lead is connected to the wrong location (I have described the mechanism that causes this before ¨C search the archives on isolation transformers, or read the 16 page tutorial I wrote while at Tek ¨C "differential measurements- a primer".
If the user does not currently own an isolation transformer and is looking at buying one for $50 or more, I would suggest an alternative ¨C a good differential amplifier. If you often have the need to measure signals elevated from ground, or the primary circuits in power supplies, I would suggest you consider a differential amplifier such as a 7A13 with an inexpensive 7K scope such as a 7603. If your main scope is a 54x series, a type W works. A good pair of divide by 10 differential probes to go in front of it is a good addition. Yes, it will cost a bit more than an isolation transformer, but both you and the device under will remain safe.
Steve
