A separate or common earthing?Which is safest?

[Ayobami Olabanji]

Safety and functionality are of high priorities to engineers.I am trying to figure out the safest of these two earthing methods for the LV Transformer and other part of the electrical system in a structure.

(i)The LV transformer to be on the same earthing system for the structure,meaning that it will be on the same earth bar as the MLV Panel,SMB, and all the DB's.A common earth pit for all the electricity supply and distribution.

(ii)The LV transformer to be on a separate earth pit located close to the transformer and another earth pit to serve the MLVP,SMB and all the DB's.

 

[roger]

All earthing should be at one location or connected together, this will reduce the possibility of circulating currents in ground loops from all the sub systems.

Roger

 

[dbuckley]

The answer is that it depends on the situation. Most of the time common earthing makes great sense, but sometimes its just downright dangerous.

Of course, the NEC insists you always adopt common earthing, but many other juristictions allow other schemes.

 

[roger]

The answer is that it depends on the situation. Most of the time common earthing makes great sense, but sometimes its just downright dangerous.

Care to elaborate on the "downright dangerous" part?

Roger

 

[dbuckley]

Gladly.

Ground is great as long as it is common, but sometimes a ground gets extended from outside of where it can safely be. Any time you hear anyone talking about stray currents you've usually got a ground to ground potential difference where you would rather there wasn't one. Ground to ground potentials are a real problem because none of the protection devices we employ stop (or even detect) ground to ground shock hazards.

One quick example - an installation on a dock in water, where the service entrance is some distance away. At the service entrance, ground and neutral are the same, but by the time the EGC reaches the dock there will be a measurable potential between the EGC and the water. This is often enough to be felt and there are many cases of it causing reported shocking. Sticking in a supplemental ground rod or two at the dock makes a bit of a difference on the meter, but doesn't make the problem go away.

What does make the problem go away is an isolating transformer at the dock, with the secondary not bonded to the supply. Stick in a couple of local ground rods, and bond them to secondary centre and you have a neutral. Legislate that all loads are fed using GFCIs. Result - a safe installation with no tingles.

This also pops up in the agricultural setting, cows getting tingles from drinking water supplies. Cows dont like the tingle on the tongue, so they drink a lot less, so their milk output drops significantly. The same technique of an isolating transformer and a new local ground is used in most countries to make the problem go away and get the production back up.

 

[steelersman]

Gladly.

Ground is great as long as it is common, but sometimes a ground gets extended from outside of where it can safely be. Any time you hear anyone talking about stray currents you've usually got a ground to ground potential difference where you would rather there wasn't one. Ground to ground potentials are a real problem because none of the protection devices we employ stop (or even detect) ground to ground shock hazards.

One quick example - an installation on a dock in water, where the service entrance is some distance away. At the service entrance, ground and neutral are the same, but by the time the EGC reaches the dock there will be a measurable potential between the EGC and the water. This is often enough to be felt and there are many cases of it causing reported shocking. Sticking in a supplemental ground rod or two at the dock makes a bit of a difference on the meter, but doesn't make the problem go away.

What does make the problem go away is an isolating transformer at the dock, with the secondary not bonded to the supply. Stick in a couple of local ground rods, and bond them to secondary centre and you have a neutral. Legislate that all loads are fed using GFCIs. Result - a safe installation with no tingles.

This also pops up in the agricultural setting, cows getting tingles from drinking water supplies. Cows dont like the tingle on the tongue, so they drink a lot less, so their milk output drops significantly. The same technique of an isolating transformer and a new local ground is used in most countries to make the problem go away and get the production back up.

nice explanation dbuck.

 

[roger]

Dbuckley, the OP is asking about a single structure, not seperate structures such as buildings and docks with considerable space between them.

Roger

 

[brian john]

Gladly.

Ground is great as long as it is common, but sometimes a ground gets extended from outside of where it can safely be. Any time you hear anyone talking about stray currents you've usually got a ground to ground potential difference where you would rather there wasn't one. Ground to ground potentials are a real problem because none of the protection devices we employ stop (or even detect) ground to ground shock hazards.

Actually there are devices that do protect against these types of problems, a ground relay. Seldom employed in the system you note but available none the less.

I also agree with Roger in regards to the original OPs question. And in that case a common ground is the only NEC viable option.

 

[dbuckley]

I also agree with Roger in regards to the original OPs question. And in that case a common ground is the only NEC viable option.

I also agree with both those statements.

Doesn't alter the underlying truth of the analysis though - there are situations where having a struture not bonded to an incoming ground potential is a better option.