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Copy edited (e.g. ref. <http://english.stackexchange.com/questions/15953>).
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Peter Mortensen
Peter Mortensen
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What you're asking about is exactly the problem with "isolated systems" and why we don't use them much.

It seems clever at first to simply have the "live" and "neutral" wires float with no reference to ground. I call this situation "First ground-fault is free" (has no consequences), as in your second illustration where the human is the first ground fault.

The problem is, it also blinds us to the first ground fault. It's free, so we do not realize it is happening. That equipment has faulted from live to ground, and we don't know. So when the person also makes a connection between, let's say neutral and ground, they become the second ground fault and it is most definitely lethal to them!

Did you catch the part about neutral being live? I actually had that happen in a building served by its own transformer. It was an isolated system by accident; its neutral-ground bond was faulty. The "first free ground fault" had happened inside a lamp on L1 phase. This pulled L1 to ground, making it neutral. "The wire formerly known as neutral" was now 120V120 V to ground, and L2 was 240V240 V to ground. All the other insulation held, and everything worked normally. And I was working on a circuit that I positively definitely turned off at the breaker, flashed L1 to ground with my screwdriver while looking away (up at the light), and surprise! The fluorescent light struck its arc, and turned back on! (not the arc flash I was expecting). I figure it out PDQPDQ, but holy smoke... I was about to touch "The wire formerly known as neutral"! Good thing I checked.

When isolated systems are used (on purpose lol), it's in facilities with active engineering supervision, where you can get away with a lot of stuff because competent staff is checking for faults at frequent intervals. As a result, "The first ground fault" gets caught early, and eliminated, before a second ground fault has a chance to happen.

What you're asking about is exactly the problem with "isolated systems" and why we don't use them much.

It seems clever at first to simply have the "live" and "neutral" wires float with no reference to ground. I call this situation "First ground-fault is free" (has no consequences), as in your second illustration where the human is the first ground fault.

The problem is, it also blinds us to the first ground fault. It's free, so we do not realize it is happening. That equipment has faulted from live to ground, and we don't know. So when the person also makes a connection between, let's say neutral and ground, they become the second ground fault and it is most definitely lethal to them!

Did you catch the part about neutral being live? I actually had that happen in a building served by its own transformer. It was an isolated system by accident; its neutral-ground bond was faulty. The "first free ground fault" had happened inside a lamp on L1 phase. This pulled L1 to ground, making it neutral. "The wire formerly known as neutral" was now 120V to ground, and L2 was 240V to ground. All the other insulation held, and everything worked normally. And I was working on a circuit that I positively definitely turned off at the breaker, flashed L1 to ground with my screwdriver while looking away (up at the light), and surprise! The fluorescent light struck its arc, and turned back on! (not the arc flash I was expecting). I figure it out PDQ, but holy smoke... I was about to touch "The wire formerly known as neutral"! Good thing I checked.

When isolated systems are used (on purpose lol), it's in facilities with active engineering supervision, where you can get away with a lot of stuff because competent staff is checking for faults at frequent intervals. As a result, "The first ground fault" gets caught early, and eliminated, before a second ground fault has a chance to happen.

What you're asking about is exactly the problem with "isolated systems" and why we don't use them much.

It seems clever at first to simply have the "live" and "neutral" wires float with no reference to ground. I call this situation "First ground-fault is free" (has no consequences), as in your second illustration where the human is the first ground fault.

The problem is, it also blinds us to the first ground fault. It's free, so we do not realize it is happening. That equipment has faulted from live to ground, and we don't know. So when the person also makes a connection between, let's say neutral and ground, they become the second ground fault and it is most definitely lethal to them!

Did you catch the part about neutral being live? I actually had that happen in a building served by its own transformer. It was an isolated system by accident; its neutral-ground bond was faulty. The "first free ground fault" had happened inside a lamp on L1 phase. This pulled L1 to ground, making it neutral. "The wire formerly known as neutral" was now 120 V to ground, and L2 was 240 V to ground. All the other insulation held, and everything worked normally. And I was working on a circuit that I positively definitely turned off at the breaker, flashed L1 to ground with my screwdriver while looking away (up at the light), and surprise! The fluorescent light struck its arc, and turned back on! (not the arc flash I was expecting). I figure it out PDQ, but holy smoke... I was about to touch "The wire formerly known as neutral"! Good thing I checked.

When isolated systems are used (on purpose lol), it's in facilities with active engineering supervision, where you can get away with a lot of stuff because competent staff is checking for faults at frequent intervals. As a result, "The first ground fault" gets caught early, and eliminated, before a second ground fault has a chance to happen.

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Harper - Reinstate Monica
Harper - Reinstate Monica
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What you're asking about is exactly the problem with "isolated systems" and why we don't use them much.

It seems clever at first to simply have the "live" and "neutral" wires float with no reference to ground. I call this situation "First ground-fault is free" (has no consequences), as in your second illustration where the human is the first ground fault.

The problem is, it also blinds us to the first ground fault. It's free, so we do not realize it is happening. That equipment has faulted from live to ground, and we don't know. So when the person also makes a connection between, let's say neutral and ground, they become the second ground fault and it is most definitely lethal to them!

Did you catch the part about neutral being live? I actually had that happen in a building served by its own transformer. It was an isolated system by accident; its neutral-ground bond was faulty. The "first free ground fault" had happened inside a lamp on L1 phase. This pulled L1 to ground, making it neutral. "The wire formerly known as neutral" was now 120V to ground, and L2 was 240V to ground. All the other insulation held, and everything worked normally. And I was working on a circuit that I positively definitely turned off at the breaker, flashed L1 to ground with my screwdriver while looking away (up at the light), and surprise! The fluorescent light struck its arc, and turned back on! (not the arc flash I was expecting). I figure it out PDQ, but holy smoke... I was about to touch "The wire formerly known as neutral"! Good thing I checked.

When isolated systems are used (on purpose lol), it's in facilities with active engineering supervision, where you can get away with a lot of stuff because competent staff is checking for faults at frequent intervals. As a result, "The first ground fault" gets caught early, and eliminated, before a second ground fault has a chance to happen.