Earthing concern

[tissparks]

Hi all, first post, but have been reading a while, so here goes. I have three HV/LV transformers feeding into three seperate switchboards but can be interconnected with each other if tx is down. The star points are fed to the switcboard NE link bar. The earth off all three swithboards then goes to an earth bar within the room. Thing is, earth bar is not connected to earth. Its only connected to the HV swithboard and transformer casings. Looked at the plans and the earth bar should be connected to an earth nest and connected to the building steel work. Before i raise the issue (apparently this wont be the first time) wanted some opinion from more experienced people. Thanks for looking.

 

[snowhead]

Is there not an earth bar near the Transformers?

 

[tissparks]

There is an earth bar at the switchboard that is linked to neutral, this is then connected to the lv db's and motor starters etc within the room. This bar is connected at each switchboard is also connected to the earth bar ( that should be according to plans connected to earth) in the room. Hope it make sense.

 

[tissparks]

To make it easier to understand, cause I'm finding it difficult myself, the question is none of the installation is earthed to ground so this seems like a problem, is it?!?!

 

[Knobhead]

Where is the HV earth nest? It’s not unusual to use the HV nest for the LV via the HV cabling. I didn’t say I like it but it happens.

The last LV switchroom I installed had no earth nest as such.

Along side each of the 11KV PILCSWA was a 70mm² back to the 11KV switchroom’s earth nest.
Three 1000KVA transformers feeding the main LV board.
From the MET I ran two 95mm² to the steelwork at each end of the plant.
For the cable to get to the switchroom they had to pass under a three lane road and two railway lines. I had four 6” welded steel pipes (one spare) run to protect the cables. Where they entered the cable chamber I welded 16mm studs on to them and linked them to the MET.
The foundations of the switchroom were concrete, I bonded the ReBar to the MET to form an Ufer earth.

Officially there was no “earth nest” ? ? ? ? Like to bet.

Bonding to the plant steelwork is to me is the most important thing.
Where is a fault going to occur? On the plant!

If nothing else. Get the steelwork bonded to the MET!


A bit off topic:
How do you run the transformer neutrals normally? At the main intake they should be linked neutral.
E54 and I have disagreed on out going feeders, I’ve always worked with 3 pole switching with a solid neutral link, E54 4 pole.
There’s arguments for and against each.

BTW, I used to cary a bit of string in case my belt and braces broke

 

[Engineer54]

Hi all, first post, but have been reading a while, so here goes. I have three HV/LV transformers feeding into three seperate switchboards but can be interconnected with each other if tx is down. The star points are fed to the switcboard NE link bar. The earth off all three swithboards then goes to an earth bar within the room. Thing is, earth bar is not connected to earth. Its only connected to the HV swithboard and transformer casings. Looked at the plans and the earth bar should be connected to an earth nest and connected to the building steel work. Before i raise the issue (apparently this wont be the first time) wanted some opinion from more experienced people. Thanks for looking.

So what's earthing the neutral point of the LV TX's at the switchboard?? Are you absolutely sure there is no earth nest (that's being used in a dual role) of any description on the MV side of things, as i am finding that hard to believe.

I'm also having a hard time of your description of having ''motor starters and DB's within the room''. Are you saying that the Main MV and LV Switchboards are not contained in it's own or separate rooms?? By the way, are the TX's mounted externally or internally??

Also not quite understanding your Switchboard layout, if the bus-bars are solidly connected through Bus couplers, then it's a single Switchboard not three Switchboards. Which would give you in the way of main interlocked breakers (manual or Automatic), 3 incomers and 2 bus couplers.

Any chance of you posting up some photo's of these MV and LV arrangements you're talking about??

 

[tissparks]

Tony, i will check for a HV nest. There is extra cabling (not on drawings) linking HV switchboard to main earth bar but not hopeful of a HV nest because the HV sub and LV sub are next to each other and there are no inspection pits in the area. I will look tomorrow.

54, this is my concern that there is no earthing of the neutral point. The TX's are caged outside at the back of the HV and LV subs. The LV sub is within a MCC that has the LV sub (switchboards 1,2,3 being fed separately from the TX's but can be connected through bus couplers if required). Also within the MCC are the DB's for LV dist and all the starters for the machinery within the building).

The drawings indicate two nests, one for each sub. But they also show the building steelwork bonded to the main earth bar at two points........there are no nests and there is no bonding. I noticed it because someone was asking for information on the external loop impedance figure at build from drawings, and then i saw the main earth bar had a link at each end but was not connected to anything other than the TX's and HV switchboard. Raised it with a manager and got a reply that they were aware of that issue and it was further down the priorities!!!!! So i am doing some investigative work to bring it back to the top of his pile. I just want to make sure i get it all right before landing it on his desk. My background is maintenance not installation hence the post for your thoughts. I find it hard to believe because this can't be safe!!!!

 

[benji]

i would have thought if the neutral point wasn,t earthed they would be concerned, it would also be a bit dark

 

[Engineer54]

Tony, i will check for a HV nest. There is extra cabling (not on drawings) linking HV switchboard to main earth bar but not hopeful of a HV nest because the HV sub and LV sub are next to each other and there are no inspection pits in the area. I will look tomorrow.

54, this is my concern that there is no earthing of the neutral point. The TX's are caged outside at the back of the HV and LV subs. The LV sub is within a MCC that has the LV sub (switchboards 1,2,3 being fed separately from the TX's but can be connected through bus couplers if required). Also within the MCC are the DB's for LV dist and all the starters for the machinery within the building).

The drawings indicate two nests, one for each sub. But they also show the building steelwork bonded to the main earth bar at two points........there are no nests and there is no bonding. I noticed it because someone was asking for information on the external loop impedance figure at build from drawings, and then i saw the main earth bar had a link at each end but was not connected to anything other than the TX's and HV switchboard. Raised it with a manager and got a reply that they were aware of that issue and it was further down the priorities!!!!! So i am doing some investigative work to bring it back to the top of his pile. I just want to make sure i get it all right before landing it on his desk. My background is maintenance not installation hence the post for your thoughts. I find it hard to believe because this can't be safe!!!!

I always insist that all earthing and or bonding conductors connected to a MET on all medium to large installations be identified at the MET earth bar, as well as any earth conductors connected to the MV and LV switchboard earth bars..


I'd still like to see some photo's of this MV and LV Switchboards and TX installation, if at all possible??

 

[Knobhead]

Unfortunately not many MET’s do get marked up.

N→E has been a nightmare over the years for me, all down to people not appreciating its importance.
I got called to a fault where two other electricians had removed the N→E link cable to the MET to make it easy to get a new cable in the switchboard despite it being clearly labeled.

 

[Engineer54]

Unfortunately not many MET’s do get marked up.

N→E has been a nightmare over the years for me, all down to people not appreciating its importance.
I got called to a fault where two other electricians had removed the N→E link cable to the MET to make it easy to get a new cable in the switchboard despite it being clearly labeled.

Usually the N-E link in main LV Switchboards is via a copper bus bar. Don't think i've ever seen the link being made by a cable. Unless earthing and bonding conductors are labeled at these larger sized MET's and Switchboards, it can be one hell of a job tracing them out once they disappear into ducts, or as sometimes the case, ....concrete!! We use split removable linked earth bars, which are also labeled as to what each section has been designated....

 

[tissparks]

Think this is a case of an incorrect install and not marking Tony, and yes 54, the N-E link is in main LV switchboard via a copper bus bar. I have a picture of the main earth bar and because it is not marked i cant say 100% what the top two cables are but i suspect they are the bonding to another main earth bar to a switchboard in another building and yes this other switchboard has two copper tapes to earth. I think this is its only path to earth!!!! If you look in the picture the earth bar to the left is for C & I and has two tapes attached, i think these have been incorrectly placed on the wrong earth bar at build. The other cables on right are tx's tank, switchboard earth bar and hv switchboard earth bar. These i'm certain because i can trace and there marked.

 

[Engineer54]

Well that earth bar arrangement isn't anything to write home about is it!! And the bar to the right has 3 X 1 MVA TX's connected to it, ...personally i'd say that earth bar could well be undersized if it's combining both MV and LV earthing systems...

Not conversant with the term ''C&I'' can you enlighten me please?? Maybe i'm having a blonde moment!! lol!!

 

[tissparks]

Undersized, not marked, not connected to earth, I've now put it back to management and recorded it in to the site log book.
C&I is a term we use for Controls & Instrumentation, might just be a term our company uses. That earth bar on the left is for a clean earth but strange that the controls guys alway say we don't have a clean earth, seems we have, I'm sure someone has connected those copper tapes to the wrong bar.

 

[Engineer54]

The term ''Clean Earth'' is a bit of an old fallacy to be honest. For IT systems it's always best to create it's own TT system, to ensure as far as is possible, that the earthing system is not influenced by the general so-called ''DIRTY'' utility earthing. Fact of the matter is, IT installations tend to create it's own rubbish and dumps it on to it's CPC's. You can sometimes actually measure more noise and crap, on a so-called clean earth, than on the utility earthing system!!