110v vs 240v on building sites.

[mattysupra]

Can someone please tell me why building sites use 110v?

Why dont they use 240v with RCD protection?

Im not a sparky so i dont know but surely a 240v system with protection is safer than 110v with out rcd protection?

 

[westie101]

The 110V is actually 55-0-55 using a centre tapped transformer, this way the maximum voltage to earth is 55V, this is classed as intrinsically safe as at that voltage there will be insufficient current flow through a human body to be lethal.

With the RCD there is always a risk of the device failing in a harsh environment so it could give a false sense of security

 

[mattysupra]

o, ok that makes sense.


So to check im clear then, the max shock i can get is 55volts then? So if i was to cut through teh cable i would only get 55volts?

If i was to cut through a 240v cable i would get a 240v shock?

 

[westie101]

That's about it, yes. That though is voltage to earth, if you were to make contact with the live and neutral at 110V you would get a shock at that voltage

 

[OwainDIYer]

I'm not sure but I think 55-0-55V predates the widespread use of RCDs.

 

[mattysupra]

so then, how come 240v is not ....120 - 0 - 120 ??

Would that not be safer in our homes???

In country's with 120v is it the same setup or the same as our 240v?

 

[JBR]

so then, how come 240v is not ....120 - 0 - 120 ??

Would that not be safer in our homes???

In country's with 120v is it the same setup or the same as our 240v?

Or, better still, why not 55V - 0 - 55V? (Accepting, of course, that the cost of changing everything would be prohibitive.)

 

[mattysupra]

so then, how come 240v is not ....120 - 0 - 120 ??

Would that not be safer in our homes???

In country's with 120v is it the same setup or the same as our 240v?

Or, better still, why not 55V - 0 - 55V? (Accepting, of course, that the cost of changing everything would be prohibitive.)

good point !

 

[bernardgreen]

Or, better still, why not 55V - 0 - 55V? (Accepting, of course, that the cost of changing everything would be prohibitive.)

At half the voltage the current would be doubled for the same amount of power delivered to the appliance hence thicker cables woud be needed.

Doubling the current in a cable increases the energy wasted in the cable by four times ( W = I squared times R ). So cables would need to be four times the size, or distance to substation much shorter hence in 110 volt countries these are very common.

with 11Kv strung along the streets at the top of the poles.

 

[wingcoax]

Not forgetting the fact that 110v tools do not get stolen at the same rate as 240v, as they cannot be used at home without a transformer.

 

[viewer]

I think that the USA does have a 120-0-120 system using two phases for larger houses.

 

[bernardgreen]

I think that the USA does have a 120-0-120 system using two phases for larger houses.

They do, even small houses often have two phases at 180 degrees to supply heavy loads such as cookers and electric water heating. ( three phase supplies are 120 degrees to each other )

One phase of the 11 kv is transformed to 220 v and this 220 v secondary is then centre tapped to create two 110 v supplies.

I saw poles feeding houses and shop along a road on the outskirts of one US town with ( I assume ) 32 kv three phase strung above ( I assume ) 11 kv single phase, tin cans of every post and at intervals double poles with a large oil cooled transformer. These took one phase of the 32 kv and fed the next section of the 11 kv lines.

 

[ericmark]

The higher the voltage the less chance of fire and the lower the voltage the less chance of a fatal shock.

The 55 - 0 - 55 or with three phase 63 - 0 - 63 is safer if done correct. This means a fuse or MCB on every line. However the Yellow brick transformer is rather dangerous in the because the fuse or trip is on the input not the output. So a 3kW brick can deliver around 55 amps to earth without tripping. The standard 1.5mm sq flex will not stand this current and will melt.

So the large 7.5 KVA transformer is safer but the 3 KVA yellow brick is not.

The system is safe as the brick should have fuses/MCB's plus RCD on the output but these rules are not obeyed.

 

[JBR]

Not forgetting the fact that 110v tools do not get stolen at the same rate as 240v, as they cannot be used at home without a transformer.

And the 240V to 120V transformers don't get nicked because they are so bloody heavy! I once did my back in trying to life one (incorrectly, it has to be said, by bending forward).

 

[JBR]

I think that the USA does have a 120-0-120 system using two phases for larger houses.

They do, even small houses often have two phases at 180 degrees to supply heavy loads such as cookers and electric water heating. ( three phase supplies are 120 degrees to each other )

One phase of the 11 kv is transformed to 220 v and this 220 v secondary is then centre tapped to create two 110 v supplies.

I saw poles feeding houses and shop along a road on the outskirts of one US town with ( I assume ) 32 kv three phase strung above ( I assume ) 11 kv single phase, tin cans of every post and at intervals double poles with a large oil cooled transformer. These took one phase of the 32 kv and fed the next section of the 11 kv lines.

Ah, that explains it. When visiting a relative in New England, I noticed her meter outside the house clearly stated 220V and couldn't understand why when her supply was 110V.