equipment grounding conductor and equipment grounding

[vanderkoppel]

We have an equipment which starts with a power distribution unit which requires 3 phase and an isolated ground.
The isolated ground goes to a ground bar in the power distribution unit to which the different units of the equipment are hooked up.
Next to this runs a so called NEC safety ground and metal conduit ground which is connected to the casing of the power distribution unit.
Now according my US colleagues according NEC the ground bar inside the power distribution unit should be connected to building steel.
I would expect the NEC safety ground need to be connected to building steel.
If I understood Mike's video's correct (NEC systems and equipment grounding) the NEC safety ground is equipment grounding and also building steel is part of equipment grounding.
The isolated earth is the equipment grounding conductor and should not be used for equipment grounding.

 

[petersonra]

The NEC uses the following ground terminology for conductors:
Equipment grounding conductor (EGC)
Grounding electrode conductor (GEC)
Grounded conductor

Grounded means a connection to a grounding electrode system. Building steel can be a grounding electrode (GE), but there are some conditions in the code it has to meet to be a GE.

Isolated ground is not a term we use much. The code allows for an insulated ground but the insulated ground still has to be connected to the EGC at some point so it is not really "isolated".

We do not use the term "safety" ground.

A GE is specifically prohibited from being part of the ground fault path so it cannot be part of an EGC, although a GEC is often times effectively in parallel with one or more EGC.

 

[don_resqcapt19]

...
The isolated earth is the equipment grounding conductor and should not be used for equipment grounding.

"Isolated earth" is not permitted for any type of grounding in the NEC. The "instrument" or "isolated" or "clean" or what ever you chose to call it, must be connected to the electrical grounding system at some point.

Only an Equipment Grounding Conductor (ECG) can be use as the "safety" or fault clearing grounding connection. The EGC is required to be run with the power conductors for that circuit. Conduits are permitted to serve as the required EGC.

 

[charlie b]

This is going to be difficult to resolve, because the terminology is inherently confusing. But let me start by saying that, for the most part, no piece of equipment needs to be connected to building steel. But let's try to get the situation clear in everyone's mind. It sounds to me like you have a "box of stuff" (you called it a power distribution unit) that has a power source coming to it (i.e., from a nearby power panelboard), that alters the electrical characteristics in some way (e.g., takes the incoming 230 volts and 50 hertz and converts it to 120 volts 60 hertz), and that then provides power to other devices that are plugged into the unit. Is that much right?

If so, then the incoming power conductors need to include an "Equipment Grounding Conductor," which is typically has green insulation, and is connected somewhere within the unit to the case of the unit. I have seen that conductor described as a "safety ground." For the convenience of the items getting power from your box, it makes sense that it would have a ground bar, and that the things powered from it would have their "safety grounds" attached to that same bar. But that bar does not get connected to building steel.

Is this making sense? Is it helping?

By the way, I was just in your neck of the woods a couple weeks ago. Here is one of my favorite photographs from that trip:
http://www.pbase.com/cbeck/image/159083619

 

[augie47]

..................................

:thumbsup:

 

[electricalist]

Is there someone avaliable that if familiar with the equipment or the electrical system then consult them.
The grounding of an electrical system has different functions ...Grounding ,method of egc and bonding can easily be done wrong and not fail right then.
Use caution..

 

[vanderkoppel]

First of all thank you all for your replies. Some things are already more clear to me (I hope).
I agree with Charli B that the terminology is confusing, but after your replies and after the definitions given by Petersonra more clear now.
What I called isolated ground is in fact the Equipment Grounding Conductor (EGC), used for fault clearing.
This is a green wire and runs together with the power wires. They call it isolated because the wire comes straight from the transformer and should not be connected to building steel.
The NEC safety ground is wrong terminology, in fact this is Grounding Electrode Conductor (GEC) to my opinion.
This is the grounding of the casing of the panels / cabinets and also of the metal conduits between the panels through which the power wires and the EGC runs.
From all the replies to me it is clear that one should not connect this Equipment Grounding Conductor to building steel.
Or as Petersonra writes: 'A GE is specifically prohibited from being part of the ground fault path so it cannot be part of an EGC, although a GEC is often times effectively in parallel with one or more EGC'.
But if all this I write is correct the naswer from Charlie B is a bit confusing.
Charlie writes: 'If so, then the incoming power conductors need to include an "Equipment Grounding Conductor," which is typically has green insulation, and is connected somewhere within the unit to the case of the unit'.
If you connect the EGC to the case of the unit and the (case of) unit is also connected to a GEC via the metal conduits isn't that incorrect? You are now making a connection between EGC and GEC, what is not allowed (assuming my conclusion about NEC safety ground being GEC above is correct).
And what has priority? A casing of a panel often is part of the EGC (in case a power wire makes contact with the case).
But now via the metal conduits this panel is also connected to GEC. So should GEC not be used in this case?
(At Charlie b, hope you enjoyed Amsterdam.)

 

[kwired]

Grounding electrode system purpose is to make a reference to earth for the "grounded" conductor of the system, and to carry transient currents to earth like when there is a lightning strike - either direct current from the strike or indirect current from a nearby strike.

Equipment grounding is tying non current carrying metal components of equipment containing electric components together so that it will create a low impedance current path and this EGC is at some point connected to the grounded conductor of the system - usually at or near the source, this low impedance path allows a high level of current to flow should an ungrounded conductor fault to the non current carrying components. You want a high level of fault current to be able to flow so that it will result in quick response of overcurrent protection devices. This will limit the exposure time of "energized" equipment frames or other non normally current carrying conductive metal parts from introducing electric shock hazards by causing a quick response by the overcurrent device to open a portion of the circuit supplying the voltage.

Equipment grounding conductors and grounding electrode system are connected together and to the grounded conductor of the system for grounded systems which kind of puts them at same potential - but certain fail modes along with the intended purpose of each portion of the whole picture are what determines why they need be installed to the rules that have been set.

Of course there are times we like to make this even more confusing like when we have a process that maybe creates more hazards from unexpected shut down then exists if there is a ground fault. That is where ungrounded systems and high impedance grounding systems come into play - but they do have monitoring equipment to still tell us there is a fault and it should be investigated ASAP or a second fault can introduce mayhem.


Add: Isolated ground is a term that mostly applies to data handling equipment and is permitted to provide a equipment grounding path that doesn't directly bond to other objects in the path back to the source, but is still indirectly bonded to those paths. The whole idea is that some circulating currents in the path can introduce data errors but a clean path to earth seems to eliminate such currents/noise and results in less data errors. I think a lot of today's data handling equipment is using different methods that are more immune to such data errors and you don't see as much "isolated ground" requirements as you once did.

 

[Smart $]

Picture worth a thousand words:

Technically, an isolated ground conductor is still an EGC (i.e. IEGC). The only difference is it is not bonded to non-current-carrying metal parts in between origin? and outlet(s). Where metallic wiring methods (metallic raceways, armored cable, etc.) are used, both EGC and IEGC are required.

?The origin is typically the EGC side of the MBJ or SBJ?, but design spec's may put origin closer to outlet(s).

?SBJ - system bonding jumper; serves same purpose as MBJ, only it is for separately derived systems on premises (e.g. secondary of isolation transformers).

 

[kwired]

Picture worth a thousand words:

Technically, an isolated ground conductor is still an EGC (i.e. IEGC). The only difference is it is not bonded to non-current-carrying metal parts in between origin? and outlet(s). Where metallic wiring methods (metallic raceways, armored cable, etc.) are used, both EGC and IEGC are required.

?The origin is typically the EGC side of the MBJ/SBJ?, but design spec's may put origin closer to outlet(s).

?SBJ - system bonding jumper; serves same purpose as MBJ, only it is for separately derived systems on premises (e.g. secondary of isolation transformers).

Now supply those loads with metal raceways or provide any other conductive path between them and you can still legally install an isolated equipment grounding conductor, but kind of don't really have a true isolated ground to that one piece of equipment.

 

[don_resqcapt19]

Now supply those loads with metal raceways or provide any other conductive path between them and you can still legally install an isolated equipment grounding conductor, but kind of don't really have a true isolated ground to that one piece of equipment.

250.96(B) permits you to isolate the metallic raceway from the equipment in that case. You many also have to isolate the metal parts of the equipment from other things like building steel if you want to maintain complete isolation.

 

[Smart $]

Now supply those loads with metal raceways or provide any other conductive path between them and you can still legally install an isolated equipment grounding conductor, but kind of don't really have a true isolated ground to that one piece of equipment.

Depends on the equipment. If the equipment has a conductive encasement which connects, incidentally or otherwise, to anything grounded via EGC, EBJ, GES, etc., that would be correct. If it is for example double-insulated equipment, the IEGC remains intact. A common culprit to making an IEGC ineffective even to isolated equipment is grounded communication lines.

And what Don said... :happyyes:

 

[don_resqcapt19]

First of all thank you all for your replies. Some things are already more clear to me (I hope).
I agree with Charli B that the terminology is confusing, but after your replies and after the definitions given by Petersonra more clear now.
What I called isolated ground is in fact the Equipment Grounding Conductor (EGC), used for fault clearing.
This is a green wire and runs together with the power wires. They call it isolated because the wire comes straight from the transformer and should not be connected to building steel.

There is nothing that prohibits the EGC from being connected to the building steel. In fact in almost all installations there will be some type of connection between the EGC and the building steel. Often this is an indirect connection. The EGC is connected to the metal enclosure of the electrical equipment and enclosure is supported by the building steel. The code does prohibit using the building steel as the required EGC. Also the NEC does not require that the EGC be an insulated conductor. Bare EGCs are permitted by the code rules.

The NEC safety ground is wrong terminology, in fact this is Grounding Electrode Conductor (GEC) to my opinion.
This is the grounding of the casing of the panels / cabinets and also of the metal conduits between the panels through which the power wires and the EGC runs.

The NEC does not use the term "safety ground". The grounding of all non-current carrying parts of electrical equipment is to be accomplished by a connection to the EGC under the rules of the NEC. Note that a wire is not the only permitted EGC under the rules of the NEC. Metallic conduits are permitted to be used as the required EGC. 250.188 of the NEC lists the items that are permitted to be used as EGCs.

From all the replies to me it is clear that one should not connect this Equipment Grounding Conductor to building steel.

There is no requirement for the EGC to be isolated from connection to the building steel. If you are installing what the NEC calls an "isolated" EGC, then the EGC would be insulated from contact with conductive objects, except at its point of connection to the electrical grounding system and its point of connection to the electrical equipment. The point of connection of the "isolated" EGC to the electrical grounding system will be at what the NEC calls the "main bonding jumper" or in the case of a Separately Derived System, at the "system bonding jumper". These are the points on a NEC electrical system where the "grounded" (neutral) conductor, the Grounding Electrode Conductor and the EGCs are connected together. This is the only point on the system where the grounded conductor is permitted to be connected to a grounding conductor of any type.

Or as Petersonra writes: 'A GE is specifically prohibited from being part of the ground fault path so it cannot be part of an EGC, although a GEC is often times effectively in parallel with one or more EGC.
But if all this I write is correct the naswer from Charlie B is a bit confusing.
Charlie writes: 'If so, then the incoming power conductors need to include an "Equipment Grounding Conductor," which is typically has green insulation, and is connected somewhere within the unit to the case of the unit'.
If you connect the EGC to the case of the unit and the (case of) unit is also connected to a GEC via the metal conduits isn't that incorrect? You are now making a connection between EGC and GEC, what is not allowed (assuming my conclusion about NEC safety ground being GEC above is correct).
And what has priority? A casing of a panel often is part of the EGC (in case a power wire makes contact with the case).
But now via the metal conduits this panel is also connected to GEC. So should GEC not be used in this case?
(At Charlie b, hope you enjoyed Amsterdam.)

The NEC prohibits the GEC from being used as an EGC, but does not prohibit multiple connections between the GEC and the EGC.

 

[kwired]

A common culprit to making an IEGC ineffective even to isolated equipment is grounded communication lines

And that was common several years ago, now we either have wireless communication or unshielded communication lines like CAT V or VI for most newer equipment. You also don't seem to see as much isolated ground specifications or requests as you once did, and when you do is is usually carried over from years past more so then that modern equipment has much problem without it.

 

[Smart $]

And that was common several years ago, now we either have wireless communication or unshielded communication lines like CAT V or VI for most newer equipment. You also don't seem to see as much isolated ground specifications or requests as you once did, and when you do is is usually carried over from years past more so then that modern equipment has much problem without it.

Agree.

 

[vanderkoppel]

Thank you

Thank you

Again thank you all to bring some knowledge to an European.
It is more clear to me now and it was less complicated as I expected.
It seems I mixed up the term Equipment Grounding Conductor and Grounding Electrode Conductor.
I thought that the isolated ground was the EGC and the so called NEC safety ground (which runs more ot less parallel and and also consists out metal conduits) was the GEC. Wrong it is now clear to me that both are EGC.
And you are allowed to connect building steel to the EGC, but you are not allowed to use building steel as a EGC. This makes sense since building steel does not guarantuee a low impedance.
Reading all the comments it looks to me it is also allowed (looking to the drawing of Smart $) to connect the EGC line to the last box where the I-EGC is connected. Not that it is usefull, but NEC will not prohibit it.
I will create a drawing this evening (now I know this is possible) that will make my problem clear, although I am not sure if it still is a problem after all I learned. So thank you all for this.

 

[Smart $]

....
Reading all the comments it looks to me it is also allowed (looking to the drawing of Smart $) to connect the EGC line to the last box where the I-EGC is connected. Not that it is usefull, but NEC will not prohibit it.
....

There is no purpose in running an IEGC if you connect both an EGC and an IEGC to the last box.

In some cases you must connect an EGC to the last box (assuming metal). For example, when using an isolated-ground receptacle, the IEGC is connected to the receptacle grounding terminal. However, the receptacle ground terminal is electrically isolated from the mounting yoke... so it is not a self-grounding type. The box being metal is still required to be grounded and unless the box is electrically isolated from the rest of the circuit and other grounded parts, to use the IEGC to ground the box defeats the purpose of the IEGC... but you can use the IEGC if the box is electrically isolated.

 

[vanderkoppel]

I added a drawing as attachment, because via Insert Image the image showed up very small.
The discussion about isolated ground we can skip.
I understood it is allowed to have the grounding Isolated Ground and Protective Earth Ground as in the drawing. Now one can ask what is the purpose of the isolated ground, and to my opinion and to those who replied there is no purpose and in fact it is useless (the casing of the GMDU is also connected to the ground bar). But according NEC is is not prohibitted.
But the discussion is about the ground wire to building steel. In the replies before it was noted that this is allowed. But is it in this case also needed according NEC?
I think I understand why this is proposed; because the transformer needs system grounding, the building steel is used as ground conductor instead of using the ground conductor at the source of the EGC line (which can be long and give high impedance in case of lightning).
And so I think it is not prohibited. But is it required by NEC?
If I understood Mike video correct this is not required because if you get overvoltage on your system you will damage your system, but it will not give a unsafe situation.

 

[kwired]

I added a drawing as attachment, because via Insert Image the image showed up very small.
The discussion about isolated ground we can skip.
I understood it is allowed to have the grounding Isolated Ground and Protective Earth Ground as in the drawing. Now one can ask what is the purpose of the isolated ground, and to my opinion and to those who replied there is no purpose and in fact it is useless (the casing of the GMDU is also connected to the ground bar). But according NEC is is not prohibitted.
But the discussion is about the ground wire to building steel. In the replies before it was noted that this is allowed. But is it in this case also needed according NEC?
I think I understand why this is proposed; because the transformer needs system grounding, the building steel is used as ground conductor instead of using the ground conductor at the source of the EGC line (which can be long and give high impedance in case of lightning).
And so I think it is not prohibited. But is it required by NEC?
If I understood Mike video correct this is not required because if you get overvoltage on your system you will damage your system, but it will not give a unsafe situation.

What is GMDU? Why is it required to run a line to the building steel?

Does it contain a separately derived source? If so a GEC to the building steel is making more sense, though I wouldn't say you absolutely have to connect directly to building steel, some instances there is no building steel to connect to as well. If not a SDS involved I don't think such connection to building steel or any other electrode is a general NEC requirement. Size would be according to 250.66 and based on the derived system conductor size.

 

[Smart $]

...Why is it required to run a line to the building steel?

Does it contain a separately derived source? If so a GEC to the building steel is making more sense, though I wouldn't say you absolutely have to connect directly to building steel, some instances there is no building steel to connect to as well....

It appears the GMDU contain a delta-wye isolation transformer. If it's rated greater than 1kVA, the secondary neutral requires bonding to the grounding electrode system at nearest water pipe electrode or structural steel electrode (SSE).

I don't see any purpose in running the IEGC in his diagram.