3 phase earthing question ??

bucky08

just a quick question regarding earth rod in 3phase industrial installation.. how many earth rods are needed.. have worked with a few contractors doing it differently option 1. three earth rods places in a triangle each a meter apart with two earth cables brought back to earth bar and joining rod 1 to rod 2 and rod 2 to rod 3...

option 2 . just one rod and one earth cable back to earth bar...

please answer with another method if if that is what you would do...

2011

bucky08 wrote: »

just a quick question regarding earth rod in 3phase industrial installation.. how many earth rods are needed.. have worked with a few contractors doing it differently option 1. three earth rods places in a triangle each a meter apart with two earth cables brought back to earth bar and joining rod 1 to rod 2 and rod 2 to rod 3...

option 2 . just one rod and one earth cable back to earth bar...

please answer with another method if if that is what you would do...

There is no simple answer to this. What kVA supply are you talking about?
Do you think that the earth rods will be placed in hardcore / sandy / stoney ground or good topsoil / muck ?

On large electrical installations value for the maximum resistance between the main earth pit and the general mass of earth is selected. Many factors have to be considered such as the size of the installation (electrically) and the resistivity of the soil. The electrical contractor must keep adding earth electrodes and earth tape until the target value is achieved. There number of earth electrodes is irrelevant, it is the resistance value that counts.

bucky08

very good answer thank you.. main reason for asking question was to try and understand option 1 set up why the triangle set up?? what this set up really achieves or if anyone has seen or put in this set up before??

2011

There is nothing special about a triangle as such.
Most of the earth pits that I have been involved in installing were rectangular in shape with an earth electrode at each corner with several in between. From my own experience I found that the horizontal earth conductors (often bare copper tape) provided a better connection to earth altho the vertical earth rods. So in your case with three earth rods the advantage of connecting them in a triangle is that you end up with this nice horizontal conductor making contact with the ground.

2011

Here is part a drawing showing an earth pit detail:



As can be seen there is no shortage of horizontal conductors.

BrianDug

I don't have the regulations at hand but I think there is something mentioned about needing more than one earth rod if the load current exceeds 200A.

I researched optimal rod spacing spacing a few years ago, I will try dig out the information when I can.

bucky08

thank you i did a bit of a search but didnt come across that set up before...

2011

bucky08 wrote: »

thank you i did a bit of a search but didnt come across that set up before...

That might be because it is from my drawing

bucky08

2011 wrote: »

That might be because it is from my drawing

your drawing doesn't show a set up using the triangle method.. ??

2011

bucky08 wrote: »

your drawing doesn't show a set up using the triangle method.. ??

No, but it shows plenty of horizontal earth conductors.
When testing the earth pits we found this type of "mat" to be very effective.

What size installation are you dealing with?

ondarack

I am currently on a job and we have been told to bury two 100x10 copper busbars as the earth electrodes. My question is would it be better to lay them in parallel or bolt them together and make 1 long bar out of it, they are 2m each. Im thinking parallel myself but unsure about how far apart and how deep.

Jennehy

ondarack wrote: »

I am currently on a job and we have been told to bury two 100x10 copper busbars as the earth electrodes. My question is would it be better to lay them in parallel or bolt them together and make 1 long bar out of it, they are 2m each. Im thinking parallel myself but unsure about how far apart and how deep.

Are you the main contractor or sub?

2011

ondarack wrote: »

I am currently on a job and we have been told to bury two 100x10 copper busbars as the earth electrodes. My question is would it be better to lay them in parallel or bolt them together and make 1 long bar out of it, they are 2m each. Im thinking parallel myself but unsure about how far apart and how deep.

I am assuming that this is a sizable installation, in which case you are asking the wrong question. The correct question is "What is the minimum acceptable resistance that I need between the Main Earth Terminal and the general mass of earth?". Then you should keep extending the earth pit until this is achieved. The larger the electrical installation the more important this is.

Whoever is the designer of the installation should be answering that question.