Separately derived system transformer extra electrode hazard?

[dm9289]

The problem is the CMP are incompetent and don't know much about grounding particularly system grounding. The grounding electrode conductor does not need to be particularly big nor is it particularly important. The grounding electrode conductor does not carry faults, it just provides a ground reference for the grounded conductor during normal operation.

I guess the electrode is more system bonding as defined in 250.4(A)(1) lightning surges etc. to your point
I got lost a little thinking more of bonding and fault clearing which gets me back to primary side egc may be more likely to work size wise.
Was just looking at 250.66 it demonstrates your point about wire size.
So, I am back to not understanding the need for the extra electrode or hazard it presents not having it. I find if I understand the hazard things make most sense to me

 

[jaggedben]

Once again, you don't need an extra electrode, just a connection to (ultimately) the same electrode(s) as the service. Building steel or water pipe are supposed to be used if they are nearby and provide that connection.

The underlying reason is to ensure that the grounded conductor (neutral, usually) of the new system does not float at a voltage-to-ground that is much different from zero. I do think this could be accomplished by the primary EGC, but as electrofelon alluded to above, the code making panel is, er, extra careful about this for some reason.

 

[electrofelon]

, but as electrofelon alluded to above, the code making panel is, er, extra careful about this for some reason.

I respect your restraint in your choice of words

 

[dm9289]

A few things here. First no, the incoming "equipment grounding conductor" can't be used as the "grounding electrode conductor" of the separately derived system. In many, I would say, most cases it would be too small. The incoming does inadvertently connect to the outgoing, at least assumedly, as they both bond to noncurrent carrying metal throughout, including at the transformer, assuming the transformer isn't in a nonmetallic case. But they aren't joined together intentionally per any code requirement. Next, you have to have an additional grounding electrode conductor, but it can go to the same grounding electrode system as service does. Fore example, it can run back tot he ground bar you describe, or got to building steel, or to a qualifying portion of a water pipe, or even be permanently spliced to a grounding electrode conductor joining two or more of the services grounding electrodes. Just not to the equipment grounding conductor of the other system.

you got me thinking more and reading and thank you.
I got on the wrong path of thinking of the electrode as far as fault clearing which is not its 250.4(A)(1) definition more related to lightning surges etc.
Your thought also helped me learn by doing a little exercise assume
Example
480v 200amp input primary breaker 3/0 cable 250.122 grounding conductor #6 copper
120/240v 400-amp secondary breaker 500kcm cable 250.66 egc #1/0 copper
In a normal case like above not compliant

My oddball case
480v 250 amp breaker 4/0 cable yes ground is also 4/0
Heres the odd stuff 4/0 (spliced in a wireway) feeds a 24kva transformer best i can see scraping old paint of label ( i see above 250 amp breaker very bad for 24 kva transformer)
So at 24kva 240v 100 amp breaker, cable #3. so via 250.66 i could use #8 copper GEC so my set up with oddball 4/0 cable size would be ok.

 

[wwhitney]

Two tangential comments:

1) Where the rules for the GEC for an SDS would result in that GEC running to the same location at which the primary EGC originates, then rather than have the two different wires in parallel, a single wire can serve both functions by meeting all the requirements for both.

2) Where there is an earth voltage gradient present, and the GES is sufficiently spatially distributed that it is not at uniform potential (i.e. the GECs and/or bonding jumpers are always carrying some current), then if the rules for the GEC for an SDS would require connecting to the GES at a location remote from the primary system's GEC to GES connection, that could impose objectionable current on the primary EGC. In which case, would 250.6(B)(1) then allow the omission of the SDS GEC and relying on the primary EGC for earthing of the SDS? As the standard rules do result in multiple grounding connections.

Cheers, Wayne

 

[tom baker]

The problem is the CMP are incompetent and don't know much about grounding particularly system grounding. The grounding electrode conductor does not need to be particularly big nor is it particularly important. The grounding electrode conductor does not carry faults, it just provides a ground reference for the grounded conductor during normal operation.

I disagree. CMP 5 is very knowledgeable and has done much to clarify Art 250. for example a few cycles ago article 250 titled was changed to grounding and bonding. The section on grounding electrode conductor And the connection to two grounding rods was clarified that the first conductor is the grounding electrode conductor into the next grounding electrode it’s a bonding jumper. If you were a new electrician today reading article 250 he would have no problem with it. Issue we have is with older electricians I’ve been taught incorrectly and that includes myself.

 

[wwhitney]

I disagree. CMP 5 is very knowledgeable and has done much to clarify Art 250.

OK, so can you explain (perhaps on CMP5's behalf) why an SDS needs a local connection to the GES, rather than relying on a connection via the primary feeder EGC? What spatial effect is being addressed by that requirement?

Cheers, Wayne

 

[tom baker]

You are allowed to use the primary equipment grounding conductor as the grounding electrode conductor but it must meet the requirements for a ground electrode conductor meeting there are special bonding requirements at each termination or junction box and in most cases that would make the grounding electrode conductor so long it would be in effective in the event of its primary function which is to Protect from overvoltage from transformer winding failures or a lightning strike

 

[tom baker]

Here’s what Mike holt once said about transformer secondary grounding electrode conductor requirements. Originally transformers were outside and required a grounding electrode conductor connection if you look at the history of grounding this was to prevent protect from winding failures or lightning.
When transformers were moved inside due to higher voltage requirements in buildings the grounding electrode conductor requirement moved with them perhaps there is no valid reason for a grounding electrode Conductor on the secondary of a transformer but that’s the way it’s been for many years. Also grounding the secondary of a SDS does provide the voltage reference to ground.

 

[zooby]

Two tangential comments:

1) Where the rules for the GEC for an SDS would result in that GEC running to the same location at which the primary EGC originates, then rather than have the two different wires in parallel, a single wire can serve both functions by meeting all the requirements for both.

2) Where there is an earth voltage gradient present, and the GES is sufficiently spatially distributed that it is not at uniform potential (i.e. the GECs and/or bonding jumpers are always carrying some current), then if the rules for the GEC for an SDS would require connecting to the GES at a location remote from the primary system's GEC to GES connection, that could impose objectionable current on the primary EGC. In which case, would 250.6(B)(1) then allow the omission of the SDS GEC and relying on the primary EGC for earthing of the SDS? As the standard rules do result in multiple grounding connections.

Cheers, Wayne

 

[wwhitney]

Protect from overvoltage from transformer winding failures

Can you provide a circuit diagram that shows a GEC/GES connection on the secondary causing a primary side OCPD to trip on "transformer winding failure" that wouldn't occur if only the primary EGC were used? My first instinct is that there aren't any, but maybe I'm missing something.

Cheers, Wayne

 

[dm9289]

Two tangential comments:

1) Where the rules for the GEC for an SDS would result in that GEC running to the same location at which the primary EGC originates, then rather than have the two different wires in parallel, a single wire can serve both functions by meeting all the requirements for both.

2) Where there is an earth voltage gradient present, and the GES is sufficiently spatially distributed that it is not at uniform potential (i.e. the GECs and/or bonding jumpers are always carrying some current), then if the rules for the GEC for an SDS would require connecting to the GES at a location remote from the primary system's GEC to GES connection, that could impose objectionable current on the primary EGC. In which case, would 250.6(B)(1) then allow the omission of the SDS GEC and relying on the primary EGC for earthing of the SDS? As the standard rules do result in multiple grounding connections.

Cheers, Wayne

This can get detailed and confusing but its forcing me to think. My guess from old prints and exploring the entire building pad and steel is likely bonded together

 

[dm9289]

You are allowed to use the primary equipment grounding conductor as the grounding electrode conductor but it must meet the requirements for a ground electrode conductor meeting there are special bonding requirements at each termination or junction box and in most cases that would make the grounding electrode conductor so long it would be in effective in the event of its primary function which is to Protect from overvoltage from transformer winding failures or a lightning strike

If get what you are saying it seems the concern is proper connection (meaning perhaps irreversible) on the primary EGC or it somehow getting modified.

 

[dm9289]

Here’s what Mike holt once said about transformer secondary grounding electrode conductor requirements. Originally transformers were outside and required a grounding electrode conductor connection if you look at the history of grounding this was to prevent protect from winding failures or lightning.
When transformers were moved inside due to higher voltage requirements in buildings the grounding electrode conductor requirement moved with them perhaps there is no valid reason for a grounding electrode Conductor on the secondary of a transformer but that’s the way it’s been for many years. Also grounding the secondary of a SDS does provide the voltage reference to ground.

Also with Mike Holt videos 2017 I believe I got the comment no one has ever died due to improper grounding but many have died from improper bonding.

 

[dm9289]

Can you provide a circuit diagram that shows a GEC/GES connection on the secondary causing a primary side OCPD to trip on "transformer winding failure" that wouldn't occur if only the primary EGC were used? My first instinct is that there aren't any, but maybe I'm missing something.

Cheers, Wayne

I agree but good to explore if there is an instance out there maybe someone can input.

 

[hornetd]

I cannot get past the thought that Separately Derived Systems do not exist in actual practice. But that's just because I want them to be isolation transformers which they were never meant to be. My experience with wiring and running medical rescue calls at dairy farms, fresh water marinas, residential boat docks, and swimming pools has left me wanting to be able to have Isolation transformers supplying the loads in all of those occupancy types. As presently written the US NEC does not permit the construction of truly isolating transformers. Laboratory listed isolation transformers are not available in sizes large enough to serve such loads. What I want to be able to do is to avoid making a conductive connection; by any name; between the primary and secondary of a field installed transformer as a remedy to all of the present problems in the facilities named and probably several others of which I have no experience. The actual culprit in these stray neutral current situations is the North American Multi Grounded Neutral Distribution System.

At one marina I worked on I came up with a rolling Isolation transformer that could be moved to the dockside of each new arrival vessel. It was sized to provide 50 amperes which matched the ampacity of the highest amperage shore power stanchion connections in that marina.

[I DO KNOW THAT MOST INSPECTORS AND INDEED MOST ELECTRICIANS WOULD CONSIDER THIS A VIOLATION OF THE US NEC SO IT IS UNNECESSARY TO TELL ME THAT!] The trans former was wired so that the supply to the primary was exactly the same as how you would set up a Separately Derived System transformer. What was different was that there was no bonding or equipment grounding connection of any kind on the load side of the winding. None of the 3 load side taps of the secondary winding was connected to the shore-side EGCs or Neutral conductors. All that was in the load cord was one neutral conductor and 2 energized conductors at 120/240 volts 1Ø, but the cord jacket was shielded under it's exterior jacket. [I have no idea, by the way, what that cord is actually meant to be used for so don't ask.] There was literally no conductive connection between the boat using that transformer as it's supply and the shore side electrical system. The load cord was supplied through a 50 ampere Ground Fault Circuit Interrupter. If memory serves the cord cap was 14-50R with nothing connected to the Round ground pin socket.

The cord shield was connected to the secondary neutral tap of the transformer and to the metal shell of the load end cord cap containing the receptacle. The shield was used to provide a low impedance fault current path back to the neutral tap of the transformer in the event of physical damage to the cord. Such a fault would trip the50 ampere double pole GFCI circuit breaker on any current which exceeded 5 milliamperes difference between the GFCI's line and load terminals.

The process used on every newly arrived boat was to hook them up to shore power through a portable GFCI. f the GFCI did not trip it meant the transfer mechanism was transferring the neutral of the on board electrical panel from any connections to the metal on the boat to a connection with the shore line only. This meant that the boat was acceptable for direct connection to the dock power service stanchion. If the portable GFCI tripped on current leakage then the boat's operator had to choose between renting an isolation transformer to connect the boat's shore line to or signing a hold harmless agreement so that one of the marina's boat wiring technicians would change the onboard bonding arrangements. If that work could be done easily then there was no charge to do the change if the boat's operator paid a week's docking fee in advance. If the boat had a metal hull or some other feature of it's construction made that task more involved then they would be offered a price to make the change or they could rent an isolation transformer. The Isolation transformers were not cheap to assemble so the rental fee was noticeable but anyone who could afford to operate a boat large enough to need a shore line connection could afford it.

 

[electrofelon]

perhaps there is no valid reason for a grounding electrode Conductor on the secondary of a transformer but that’s the way it’s been for many years.

IMO "that's the way it's always been" is not a valid reason for a requirement and the CMP should be removing or updating some of this ridiculous stuff that no one knows why it is in there and there is no scientific justification for it. And there are plenty more in article 250 irreversible splicing for GEC's, for example.