Isn't grounding where people can touch dangerous?

[Gew]

Hey engineers!

I'm not an engineer, I know nothing, so I thought I'd ask you about this.

From what I've heard, if you don't have proper ground (a rod) in your house, you should never try to make a "cheap DIY fix" by conencting ground to some radiator or whatever. From what I've heard, the reason for this is that when some appliance in my 230v AC house breaks so that live touches the metal case, and ground trips, then any possible person in the building who happens to hold his or her hand on another radiator (or pipe) in the house will get the same shock as if he or she put two fingers in the outlet, and get electrocuted / die.

So, with this as background, I became very curious when I got to my local commuter train station yesterday, and noticed that a ground cable is attached to a friggin' metal gate on the platform, where all commuter people stands all day. This would mean, that if that ground wire was tripped by a electrical fault, then all the people who would lean against that fence that moment would get a severe shock.

How can this even be legal??


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[Rockingit]

Quite the opposite - it makes things safer.

 

[littlespark]

Path of least resistance.

If current is already flowing directly to the Mass Of Earth, then any other parallel path through something more resistive…. Like a human body… it would be negligible.

 

[Simon47]

The first thing to note is that railways have their own set of rules - especially if there's electric traction supplies. Everything will be bonded.

But within your house there are two separate things : bonding and earthing. They are often confused because they are fairly entwined in most cases.

Earthing is there to provide a fault current path and cause the supply to be disconnected if there's a fault. E.g. fan heater element touches metal case - instead of the case becoming live, fault current flows through the "earth wire" (proper name: circuit protective conductor, CPC) and the fuse blows and/RCD trips.

But, there are situations where the CPC can become live. So if you have something locally earthed (such as a metal gas or water pipe coming out of the ground, known as an extraneous conductive part) you coukd have a dangerous voltage between something earthed via the supplier's earth and the locally earthed part. To avoid this, we employ bonding from the main earth terminal to any of these extraneous conductive parts.

Bonding is not earthing, but since at least one of the bonded items is itself earthed in all but a few specialist locations, the end effect of bonding is to earth the bonded items.

Now, if you don't have an earth provided by the supplier, then you need to privide your own. This is NOT by putting a wire on a radiator !
It will involve an earth rod or tape driven into the ground or buried (other techniques exist, especially if planned prior to construction.) This needs some thought, and the ability to measure the impedance (resistance) of the earthing system.and design the rest of the system to suit - or redesign the earthing system if the impedance is too high.