Delta Secondary Ground Fault Indication Req'd? 6

[OhioAviator]

Hello everybody, and I hope someone can knock some of the rust off my brain regarding the NEC...

I recently inherited a 1500 kVA stepdown transformer, 4.16kV Delta to 480 V Delta that feeds a couple of motor control centers in an industrial plant. There are no grounding transformers on the secondary system, thereby making this an ungrounded 480 v system. In light of this, my questions is:
Is a ground fault detection system (e.g. "three-light system" or something similar), required by the NEC for this installation to alert the plant operator of a ground fault, should one occur. Right now there is no ground fault indication system installed. If it is an NEC requirement, can someone cite the particular reference in the NEC?

Thanks for your help.

 

[CHDean]

Not required but not a bad idea. NEC 230-95 requires ground fault on WYE systems =/+ 150V to ground but not over 600V phase to phase. This only applies to service entrance applications for each disconnect of 1000A or more.

A ground fault indicating light system would be a relatively inexpensive way to alert the operator of a fault although on a true delta system minor ones can be fun to track down so indicators are often ignored.

 

[OhioAviator]

Thank you CHDean. I couldn't find the reqrmn't for it in the NEC either, hence my posted question.

My first preference is to install a zig-zag transformer grounding system, but that is obviously more expensive and will be viewed as a "production inhibitor" when a 480 volt circuit trips somewhere. That's what I'm gonna' propose and have the ground fault indication system as a backup plan.

Again, thanks!

 

[CHDean]

A zig-zag with a pulsing circuit can be a great troubleshooting tool. They use a snap around ammeter with about a six inch window to trace the fault down. But like most things that work they aren't cheap. Good luck.

 

[Sett]

Hola OhioAviator!

The solution to your problem is a 51G relay with residual connection (the three ct's are connected in shunt with the relay)

I recommend read the IEEE C37.91-1985 for more info.

Good luck!!

 

[OhioAviator]

Hola Sett,

Thanks for your suggestion regarding the 51G relay connected in a residual fashion. I'm very familiar with that application although I haven't specified it myself. I was always under the impression (perhaps erroneously) that a current sensor type ground fault detection system such as this would not work on an ungrounded delta system since the ground fault current cannot get back to the source (at least for the first ground fault). I will definitely have a read at the ANSI/IEEE spec. you cited.

And Thank You to CHDean for suggesting the pulsing circuit fault location scheme. It sounds like a "kindler-gentler" thumper system that can be used to track down ground faults quite effectively. I'll certainly look into it.

Again, Thanks!

 

[peebee]

OA, you are correct, a CT-based relaying system will not work.

However, a PT-based system (where the 3 PT's essentially replace the 3 lights in your proposed scheme), with an "open delta" configuration will work, at least as effectively as the lights if not better. This is typically called a "59G (ground undervoltage)" relay.

See thread238-38660 for lots of details and links to schematics and catalog cuts.

In general, both the light-based sytem and the PT-based 59G system provide a high-impedance reference to ground, the lights and PT primaries are both in a grounded-Y configuration which is used to measure the voltage of each phase to ground.

 

[OhioAviator]

Peebee,

Thank you! for your suggestion on using the Basler 59G relay. I looked at the other thread and it addresses my problem exactly. This relay is what I'm looking for if I can't get the zig-zag transformer and associated relaying approved.

Again, thank you all.

 

[peebee]

By the way, Basler is not the only one who makes such a relay. I know for sure that ABB has a very similar relay, and I'd suspect the other relay manufacturers also have one. It's really not much more than a glorified undervoltage relay.

 

[mvcjr]

And you don't have to wire the 59G relay to trip the 480-V breaker. Just set it to trigger an alarm or indicator to warn you of the presence of a grounded phase conductor so that you can fix it before a second ground on another phase occurs.

God bless!

 

[govolsdb]

I am also responsible for a 480 Volt ungrounded system. I am considering installing a line insulation monitor (Cutler-Hammer D64L2). Do any of you have any experience with these? I am assuming that it would be better than the PT-light bulb arrangement.

 

[huntercon]

Our experience with Bender (

http://www.bender.org)

has been good. They have been manufacturing Insulation Monitoring Devices (IMDs) for just this specific application for over 50 years.

Does your AC system have DC components like VFDs or VSDs? If so, make sure the monitor you choose will 'see through' the inverter/converter to faults on the DC portion.

I just completed a proposal for a customer where I was able to replace 3 PTs + 59G relay with just one Bender monitor.

 

[busbar]

My favorite description of the potential damage in low-voltage ungrounded systems is:

http://6L6.net/localuser/busbar/dat/beemaIPSH6z.doc

[from the 1955 Beeman Industrial Power Systems Handbook]

An excellent paper is:

http://www.neiengineering.com/papers/paper1JN.pdf

Problems and solutions for low-voltage ungrounded systems have had extensive discussion on this board—primarily in forum 238.

 

[huntercon]

busbar: Thanks for the links to the papers/reports. Have you had any experience with online ground fault 'location' systems for 480VAC ungrounded (delta) systems in heavy industrial locations? Bender does make one, but seems to have limitations when it comes to large conductors and conduits. Their design works better with AC & DC systems with smaller conductor sizes (back-up batteries, etc.).

I stumbled upon an auto manuf. plant in Canada who (up to recently ) still used an old (30-40 year) unit which injects an 18A signal onto the system. According to them, there are new regulations on the current level used to locate grounds - something like 5-10A.

Can you shed light on this?

 

[huntercon]

...one more thing - what is 'Forum 238' and where can I find it?

 

[busbar]

2. huntercon — Electric power engineering Forum forum238

1. I have used one system and hated it. It looks good on paper, but IMO, current finds too many routes in raceways and conductors in an industrial environment. See

http://www.geindustrial.com/products/manuals/GEI-72116.pdf

http://www.stbinc.com/products/fault/ammeter.html

[low-voltage version with “horseshoe” jaws.]

http://www.sea.siemens.com/pde/swgr/LV_High_

Resistance_ Grounding _System.pdf

 

[busbar]

huntercon — Type “ ungrounded system “ or “ high resistance ground “ in the site’s advanced-search feature at

http://www.eng-tips.com/search.cfm

A related few are thread237-60429 thread238-9005 thread238-45575 thread238-74381 thread238-73684 thread238-47990 thread238-65999 thread238-64223 thread238-77191 thread237-71259

 

[MakeItSpin]

I also have inherited several ungrounded 480v delta systems at mine sites, ranging from 150kva to 2000kva. The best (and very inexpensive) monitor I've found is manufactured by Symcom Inc. Visit

http://www.symcominc.com

and check out their products.
They will do a modification of their 102A so it monitors voltage to ground (rather than line/line), the set point is adjustable and allows you to avoid problems if the phases "wander" around ground, as delta's can do. You can do anything with the N.O./N.C. contacts you want. We light a light when a phase goes to low potential to ground, when the operator sees the light, we turn loads off one at a time until the light goes out. At that point you pretty much know which motor or circuit is the problem.

 

[huntercon]

MakeItSpin: Careful with using 'voltage' relays to detect grounds in ungrounded systems. That method is somewhat outdated and ineffective - especially when you are trying to identify balanced/symetrical grounds and grounds within the DC portions of VFDs and VSDs.

Not to wave their flag again, but I do know that Bender has recently been working with the mining industry and MSHA to develop a specific line of ground fault monitors for ungrounded systems in mines. Check out their mining products:

http://www.benderrelay.com/mining.htm

 

[ChuckArthur]

Check out NEC Article 250.36 item (3) which does require some type of ground detection for High Resistance Grounded (HRG) systems. I have always figured that this would apply to ungrounded systems as well since a HRG system "looks like" an ungrounded system except that it can eliminate the transient overvoltage problems if the resistor is sized properly.

Beside Bender, there is another manufacturer of ground locating equipment for ungrounded/resistance grounded systems. Check out

http://www.multi-tech-industries.com/ground_fault.htm.

I have not used one of these personally, but I have been told that they work great!