TT system bad earth help

[RogertheBodger]

OK let's say the RCD failed and you touched a piece of live metalwork.

The current you would receive and hence the shock would depend on how much of this fault current was flowing through you and how much was flowing through a low resistance path back to the transformer winding IE through the rod, through the earth, through the transformers own rod and also any other parallel paths that are available.

So you see what I am getting at, if the RCD failed then the amount of current you would receive would be dependent on the value of the resistance of the path from the fault back to the star point of the transformer, your own earth rod forms a part of this route and hence the lower the reading of Ra the lower the amount of current that would flow through you (electricity always takes the easiest low resistance path).

If the fault current has a nice easy route through the ground via a very deep earth rod then it is less likely to want to find a path back to the star point through you and hence you would receive less of a shock in the event of an earth fault.

I am sure with the right maths you could calculate exactly what this shock current would be but I'm just too knackered to try this evening.

You have a rest while we think about it lol.

TT system, the RCD is not working, if the Ra is not enough to trip the mcb and you have hold of this circuit, then I am afraid you are in trouble.

 

[GLENNSPARK]

just another thing here n all.....you say that the C/U looked in poor condition so you conducted an Ra test.....so if the C/U had been in good condition would you still have conducted an Ra test?....

 

[La Poste]

Then I am afraid you are in trouble.

The story of my life. :8:

I'm just wondering if this discussion about low Ra values is purely about bringing stability to the system or if there is a touch voltage element involved in this as well.

Are we looking at low Ra values to make the system more stable or are we looking at low Ra values to reduce the shock current we might receive in the event of a fault?

 

[Engineer54]

I have to disagree with the latter part of this statement! All the TT systems I have come across in this area (Warks) have deteriorated over the years. In some cases it's due to the reduction in height of the water table and in others the lack of maintenance of the rod, cabling and connection to the rod.
I've just had one house at 350 ohms PMEd -the rod was buried under concrete!
At another, I found the rod lying horizontally on the flower bed behind the dog kennel - didn't get too good a reading off that. On another you could slide the clip up and down the old piece of water pipe that had been banged into the ground 60 years ago.
LOL
Pete

Yes you have a valid point, if the ground conditions on a rod location change for the worse, such as drying out. But under normal conditions, deep driven electrodes will generally always improve over a given time, as the soils consolidate around the bare electrode. Especially when coupled rods are involved, as the coupler has a slightly larger diameter than the electrodes, so any rod(s) after the coupler(s), will not be in solid contact with the surrounding soil directly after installation...

How many electricians maintain the rods that they have installed?? I'd say virtually none, certainly not on domestic installations!! Come to that, how many present day electricians know how to maintain a TT system?? Again, i would have to say Not Many, most wouldn't even consider the need for maintaining a TT system!!!

 

[Engineer54]

Is that in China E54?

No, it's my house in Cyprus. I'd say that around 95% or so, of all installations in Cyprus, are TT systems here. Plus most supplies will be from overhead networks, apart from those in the built-up towns and cities where they have been converting to underground distribution.

Generally speaking, only installations that are directly fed from an in situ distribution TX such as in a commercial building or a large block of apartments will have a TN-S supply...

 

[Ponty Massive]

Come to that, how many present day electricians know how to maintain a TT system??


It's easy, just press the test button !! :cowboy:

 

[Engineer54]

Come to that, how many present day electricians know how to maintain a TT system??


It's easy, just press the test button !! :cowboy:

I'll take it that was said with tongue in cheek, ...going by your smiley!! lol!!!