Equipment Earthing, Single point vs. two or more

[Power0020]

In a substation or power plant, equipment are earthed from two points or more to provide redundant connection paths in case of losing one (cut, loose connection,...etc).

Some other practices limit the connection to equipment and foundations to one only to avoid circulating fault currents within them. (this case is similar to two point bonded cable sheaths).

any clue?

 

[Skogsgurra]

Yes. Lots.

It is, to some extent, dependent on what distribution system you have. TN, IT, TT or whatever.

In my opinion, the dreaded "circulating fault current" is coveted and beneficial. The more low impedance paths there are, the lower the potential difference between different parts of a plant. That is why you have ground grids with multiple connections from machinery to grid and also, at least in process plants, an "above-ground" grid of conducting pipes and steel structures.

The "hum pick up" that so often is referred to is usually less when you have multiple connections to a good ground grid. The ultimate version is a continuous ground plane.

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[krisys]

I have come across situations where engineers specify special insulation between the foundation bars to avoid any closed conducting loop to avoid the foundation heating. In such cases even the earthing needs some special consideration. This is specifically required for the foundations of reactor coils.

Other area for the foundation earthing/bonding limitation is where the soil is highly corrosive and the foundation is protected.

Another area for such restriction could be the location where the cathodically protected installations are present. The presence of a slab reinforcement grid may cause draining of DC currents from co-located cathodic protection systems. Under such circumstances, due attention shall be paid to corrosion and CP aspects.

Otherwise, all earthing and bonding connections between the slab reinforcement grid, foundations, driven piles, construction elements and earth plates shall be in place.

Hope this helps.

 

[karls2]

IEEE request at least 2 connections to earth grid for some MV HV equipments as Transformers MV Switchgear NER ...

 

[mls1]

Are you talking about equipment grounding or neutral bonding? Ground loops are a concern when you have multiple neutral bonds to ground forming parallel paths but multiple equipment grounds are not an issue and are in fact often required. For example, parallel feeder cables must each carry the full equipment ground conductor in each path so clearly there are parallel connections.

 

[Power0020]

Well, if a foundation rebars are connected to grid from two points, any fault within the SS will go through the foundation rebars being too much big compared with grid conductors (total CSA area of rebars), and corrosion may well be present in this case.

NZ practice puts only one conductor per foundation.

 

[squeeky]

If you are earthing from multiple points it may make testing difficult. We bring all the earth mat / systems back to an earth bar which has a removable link to a second bar where all the equipment and bonding attaches. When it comes to testing the earth mat, we split the joint and conduct the test.
Krisys. Due to electrolytic action the earth can be eaten away. I have been toying with the idea of putting in sacrificial plates of Zinc which will corrode faster and thus save the copper. I can’t get an answer from the environmental guys.
IEC 62305-3:2011, table 5 gives a table of materials with recommendations and indications of where corrosion will occur.
Karls2. We connect all our transformers with two separate earths to the earth bar. No daisy chain connections. I do not have access to IEEE standards.
mls1. We only have one point grounded at any one time. Besides the circulating currents, it can split your earth fault current and then the protection can’t see the full current and may fail to operate as designed.

Power0020. It looks like you are in New Zealand. I though you used the British regs. I would be interested to learn more about this one conductor per foundation. Please can you elaborate.

 

[Marmite]

In my experience, which is transmission and distribution utilities in the UK, usually any item of plant which can be a probable source of fault current such as an arrester, transformer, earth switch, reactor, cap bank, switchboard etc is earthed at two points. They are usually connected on opposite sides of the equipment, and each earth connection is sized to carry the full prospective earth fault current on its own. Usually the earths run in opposite directions to pick up the main earth grid. I work in Australia now, where the ENA Guide EG01 Substations Earthing Guide recommends exactly the same approach. Air cored reactors especially need careful earthing design to avoid creating loops within the magnetic field.

Regards
Marmite

 

[karls2]

@Marmite
Some times it's not only "is sized to carry the full prospective earth fault current" but sized for phases short circuit current also. (especially if you have 2 earth fault on two different phases)on the same site.

 

[Power0020]

@squeaky, they put an embedded copper strap within the equipment foundations (only one per foundation greater than 4 Sq.m), this is to avoid grid fault currents flowing through the foundations causing corrosion / heating.

this is usually used for outdoor foundations.

Buildings are excluded from this required and rebars are eathed every 5 m.( that is usually not an issue with fault current circulation since the number and size of rebars in buildings will represent a huge conductor and hence no major heating)