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Why isolating transformer should be used to protect only one item of equipment at a time?

I tried to search and I found this answer, but I could not understand it.

This is why, ideally, an isolating transformer should be used to protect only one item of equipment at a time. With one item a fault in the equipment will probably not produce a dangerous situation. The transformer has done its job. BUT with N items of equipment - if one has a fault from neutral to case or is wired wrongly this may defeat the transformer such that a second faulty device may then present a hazard to the user.

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    \$\begingroup\$ Where did you read this rule? Add a reference or link into your question. \$\endgroup\$ – Transistor Jun 19 '17 at 6:45
  • \$\begingroup\$ @Transistor - FYI I've found the quote and added a link to it. \$\endgroup\$ – SamGibson Jun 19 '17 at 10:15
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You can power multiple devices from the same isolation transformer. However, in some cases this makes the setup less safe.

Whatever is powered from the isolation transformer requires two connections to some other object (like you) for current to flow thru that object. Equipment that is powered from a unisolated source can cause shocks with only one other connection. That is because one connection has already been made to ground.

The more equipment you power from the same isolation transformer, the more chances of one accidental connection to the rest of the world, thereby making everything else less safe. This is what the paragraph you quoted seems to be referring to.

However, there are also safety advantages to powering more equipment from the same isolation transformer. You are still more protected by two connections being required before shock. If the multiple pieces of equipment need to be directly connected together anyway, then you must power them from the same isolation transformer if you want there to be any isolation.

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An isolation transformer is a transformer used to transfer electrical power from a source of alternating current (AC) power to some equipment or device while isolating the powered device from the power source, usually for safety reasons. Now independent device may have different safety ratings. (safe current, safe power etc.) So that's why it is obvious to use separate transformers. But if the case is the safety requirements for all the devices are same then you can connect multiple devices to same transformer.

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  • \$\begingroup\$ Ravi C , i think your answer is exact the main reason so if i use different equipment on the same transformer this may cause hazard , but if all equipment are the same ( e.g grinders ) this will be safety, i am talking from safety vision , also if the equipment are different the shock may occur from the difference of voltage or what ? \$\endgroup\$ – Mohamed Emam Jun 20 '17 at 4:14
  • \$\begingroup\$ There is no relation between shock and equipment. Since the current transfer takes places on lowest resistance path and in this case you have touched the wire (or body) means you are the lowest resistance path, so the max. power will be transferred through your body. Isolation only lead to not damage power source by this shot circuit, or vice-virsa. \$\endgroup\$ – Ravi C Jun 20 '17 at 8:27
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To the best of my knowledge, isolation transformers are used for a couple reasons; Safety, and Troubleshooting/Testing. Both of which benefit from the isolation which decouples/isolates the Earth ground connection of the primary and the equipment/circuit ground of the secondary. The secondary becomes its own isolated loop. General practice has always been to use a single isolation transformer for each system that needs isolation because real isolation transformers come in many different configurations, usually specific to a particular piece of equipment or circuit. However, I do not see why you could not hook up more than one if you really wanted to. Just be sure both circuits meet the specifications of the ISO XFMR.

Here is a link to an article about isolation transformers.

https://www.allaboutcircuits.com/technical-articles/transformer-isolation/

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The link in your question answers why to use isolation transformers but doesn't state that only one piece of equipment should be connected to it.

As you seem to think, it should be possible to connect multiple devices to the one transformer. One reason for not doing this is that, for some reason, the device needs to be floating (isoltated) relative to the other devices to prevent electrical interaction or strange current return paths between them.

In Europe where 230 V phase to neutral is the norm portable tools are powered on 110 V, centre-grounded transformers. This gives a 55-0-55 secondary (with 110 V between the outer terminals) effectively limiting the shock hazard to 55 V with respect to ground. If either phase shorts to ground its fuse will blow. In the event that the ground connection is lost (which might not be unlikely on a building site) some reduction in risk is provided by the reduced voltage of 110 V.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. European tool transformer wiring. Note that the grounded centre-tap limits the phase voltage to 55 V AC.

In this case it is also permissible to connect multiple tools to the one transformer. The first one to fault will blow the fuse and stop them all.

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