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I'm working with isolation transformers and I need to know the concept of power utility pole mounted isolation transformer because they are related.

If the neutral of North American pole utility is not grounded below the the pole, the neutral is said to be floating (as shown below). But how can you get a shock by touching it by there is no direct path to ground?

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Remember the secondary of utility transformers are just like the concept of isolation transformers. If you don't ground the secondary, then you can safety work on equipments as there is no direct path to ground so even if you accidentaly touch one wire and ground, you won't get a shock. But how come I read reports of people getting a shock when they touch a neutral that is not grounded at the utility pole? What is the path to ground in the pole utility isolation transformers?

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    \$\begingroup\$ Since they are floating, the neutral can be at a different voltage than the ground. Even if there is no connection, the line has a potential energy stored in it, very much (or exactly) like a capacitor. Touching the line, would create a discharge and shock you. Those discharge are not fatal because they are very short but definitely not enjoyable. \$\endgroup\$
    – Damien
    Nov 23, 2018 at 11:43
  • \$\begingroup\$ But in an isolated transformer, you can use any line as hot and ground, there is no reference. The utility pole also uses isolated transformer. So if you didn't ground the neutral, can it become hot? Or can the centertap of the two 120v become hot and either one of the original 120v line become the centertap voltage? \$\endgroup\$
    – Jtl
    Nov 23, 2018 at 11:50
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    \$\begingroup\$ The Phase is what carries "the energy" and the neutral is a return line. Both have energy going through as long as there is a load connected to the end and if it's floating either can be at either potential. So if you have a load connected and the neutral is not grounded, then yes the neutral can become hot. \$\endgroup\$
    – Damien
    Nov 23, 2018 at 11:53
  • \$\begingroup\$ I have thought about this for days. About the floating neutral and how you said touching the line would create a discharge and shock you but not fatal. Is there no instance when a 13,800 volts spark can cause a person heart to stop beating or lethal? Can't any discharge kill? \$\endgroup\$
    – Jtl
    Nov 27, 2018 at 1:52
  • \$\begingroup\$ Your example was for 120VAC. There are 2 ways you can die out of an electrocution. The most common is cardiac arrest and the second is through burns. To have a cardiac arrest, you need to have an electric shock which is longer than a cardiac pulse. If it's electrostatic discharge it won't be long enough to cause a cardiac arrest. Now if you have high voltage, then it can induce burns, either on the skin or internally as the current goes through the body if the discharge is powerful enough (like lightening). \$\endgroup\$
    – Damien
    Nov 27, 2018 at 7:25

2 Answers 2

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First of all, just because the neutral is not bonded to ground at the pole does not mean that it isn't bonded to ground elsewhere, such as at the service entrance of a building.

Second, even if the secondary is completely floating with respect to ground, keep in mind that there's a significant amount of capacitance between the primary and secondary, and the primary is being driven with several thousand volts. This capacitive current can be quite significant. This is one reason that the secondary is grounded in the first place.

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  • \$\begingroup\$ How will this capactive current discharge in the secondary when the utility ground rod is gone? Will it get to any load or what situation will it gets discharged to the secondary or what lines would get the current? line to line or line to original neutral? \$\endgroup\$
    – Jtl
    Nov 24, 2018 at 1:27
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Floating neutral just means that the neutral is not bonded to the chassis. When the neutral is not bonded to the chassis, you need 2 faults to incur a shock, such as live to chassis short and then touching the chassis and neutral (or just touching neutral and standing on earth if the chassis is earthed). This is arguably more dangerous because an existing fault can go unnoticed and then just innocently touching neutral and the chassis will cause the second fault. You can earth the neutral yourself by connecting it to a consumer unit earthed busbar, and then it will basically behave like a bonded neutral generator, moreso when you earth the chassis as well.

With a bonded neutral generator, a live to chassis fault would trip the breaker. Touching live and the chassis would result in a shock. The idea is that the human fault involves touching live, which they are less likely to do.

Floating isn't quite the same as 'float' in floating pin - it's always being pulled to 0 potential in the context, and a floating ground may be being pulled to some voltage if using Y capacitors as opposed to completely detached like a floating pin. I think floating in this instance means that it is just not connected to earth, such that when you use a voltmeter to reference it to earth you don't get 0V, you either get a nonzero voltage or open circuit (high impedance).

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  • \$\begingroup\$ I would think that a line can't be called "neutral" if it hasn't been neutralised by earth bonding. \$\endgroup\$
    – Transistor
    Apr 29, 2021 at 13:21
  • \$\begingroup\$ @Transistor I didn't realise neutral had specific connotations, I thought it was just the name of the wire. Maybe you want to call it return wire in that case \$\endgroup\$
    – Lewis Kelsey
    Apr 29, 2021 at 15:13
  • \$\begingroup\$ Yes, the neutralising makes it safe as in "neutralising the attacker". Even "return" that doesn't seem appropriate on a floating system. It's AC so both wires would be feed and return. It works if you designate a certain reference point on the system such as a named terminal of the transformer secondary. That would then be the "return" point. \$\endgroup\$
    – Transistor
    Apr 29, 2021 at 15:16

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