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This image (copied from this post) shows and explains that local power transformers are connected to ground:

enter image description here

This is why we get shocked when we touch a hot wire. Current flows through our body to the ground and back into the transformer completing a path.

Why is the transformer grounded to begin with? What happens if it isn't?

JRE
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Dan
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  • Actually the diagram is not entirely accurate - or it may depend on where you live, because different countries have different grounding systems. They way you have drawn it does not seem typical but unsafe, because at your house, neutral and earth wire should be tied together at your service inlet, because otherwise if inside your lamp the live touches the metal case the short circuit depends on soil resistance and may not blow up the fuse. – Justme Nov 11 '23 at 18:24

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Believe it or not, it's for safety. If it weren't grounded, it would "float" to quite high voltages relative to ground, mainly as the result of weather and other environmental effects. This would require enormous amounts of insulation throughout the system and make accidental shorts that much more dangerous.

Dave Tweed
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  • Why would it float tho? I would image the the inductor within the transformer would just keep the current in sync with the main power generator? – Dan Nov 11 '23 at 04:41
  • It isn't the power from the generator that's the problem. The transmission lines, and even the wires in your house, are capacitively coupled to the environment, which induces additional voltages in them. There's plenty of capacitive coupling through the pole transformer, too, which means that a lightning strike on a transmission line would cause the voltage in your house to jump by thousands of volts as well. Grounding the secondary avoids that. – Dave Tweed Nov 11 '23 at 13:03
  • I see, so when it's not grounded (i.e it's floating) any magnetic field from the surrounding area could effect the current in secondary, grounding it makes sure my house and every other property has the same voltage in their outlets. Would floating happen only when I have a closed circuit (i.e a lamp is turned on ) or it floats even when the path is open? (assuming electrons can't even move in an open path). – Dan Nov 11 '23 at 16:06
  • @Dan Even when it's open. I think much of it has to do with the fact that you are a long way away from the local transformer which means lots of wires and stuff to intercept the charges constantly moving around in the environment. Which is why isolation transformers on the desk in front of you can make things safer in some instances rather than more dangerous. – DKNguyen Nov 11 '23 at 17:43