NEC Code pertaining to Ground Fault Protection for 3 Phase Motors

[TimWilborne 108]

I have been through the NEC code and see the requirements for ground fault circuit interrupter protection for 125 volt 1 phase circuits in section 210.8, but is there any specific code that requires ground fault circuit interrupter protection on 3 phase motors in wet environments? What wiring practices to you implement when installing 3 phase motors in wet environments Thanks TW

[Duffanator 1]

We just use a three phase overload set to the amp rating of the motor times the service factor and fuses to protect the wires going out to the motor. They do have overloads with built in ground fault detection but they are more of a headache than anything else, they trip all the time for no reason at all. Our entire plant is washdown so almost every motor we have gets wet constantly. The most important thing is to make sure that the motor is properly grounded, if it is and the windings short to ground then the overload will trip out almost instantly. We also like to use IEC overloads and not NEMA because they will actually break the circuit. NEMA overloads will just trip and open up the aux. contact to shut off the contactor but if the contactor sticks then there is still voltage going out to the motor. With IEC the overload will actually break the circuit to the motor and you can also wire in an aux. to break the neutral to the contactor. (Also why it is a good idea to fuse the wires so you don't start a fire!)

[BobLfoot 302]

Tim check out the following : 230.95 - Ground Fault Protection of Equipment 250.4 - General Requirement for Grounding and Bonding 215.10 - Ground Fault Protection of Equipment 240.13 - Ground Fault Protection of Equipment

[TimWilborne 108]

Duffanator, thank you for your insight. So do you require the ground fault overloads on all motors that are installed in your plant despite the nuisance tripping? Also, this only applies to 150 to 600 volt systems of 1000 amperes or more

[BobB 58]

Ground fault nuisance tripping is the biggest pain of all time. I have seen generator earth fault relays trip when the generator is not running. Not really applicable to motors except very large ones but I much prefer differntial ptotection on alternators. Pick up earth fault much quicker than earth fault relays and no nuisance tripping. Electrical engineers take some convincing though. Have not lost the argument yet.

[Duffanator 1]

We have 3-phase motor ground fault detection overloads on, I think, 3 motors out of the 500 or so in the plant. The only other motors we have that have some kind of ground fault detection are the big 150/200 horse power air compressor motors and they are driven off of big soft starts that have ground fault detection built in. Otherwise everything is just standard overload protection and fuses for the wire current rating. Every machine we build or is built for us follows that rule so I'm assuming it's an industry standard. We've had a lot of motors short to ground but none of them have ever caused a problem other than tripping overloads and blowing fuses. Again, the main thing is that the motor is actually TIED to ground correctly, otherwise you will have a problem with the chassis of the motor possibly becoming hot or getting current leakage to ground but as long as the motor is properly grounded there should be no issue.

[BobLfoot 302]

Figured most of these were "red herrings" Tim, but I had the neat NECPLUS Subsciption and wanted to post what it found.

[paulengr 44]

I work in mining. MSHA is VERY strict when it comes to ground check and ground fault monitoring for underground mines. These are two different terms that I'll come back to. There is ground fault for equipment, and ground fault for personnel. Don't mix the two up. Personnel ground fault circuit interruptors are required in bathrooms, kitchens, outdoors in residential applications, etc. Equipment ground faults are required for high current applications, etc. At low voltages and currents (<1000 volts), you can usually use a solidly grounded delta-wye transformer. This is the most common. It works very well. You can't really measure ground fault current (except through going through a lot of gyrations) since the ground impedance path is very low. All your equipment is already bonded. So a phase-to-ground fault travels back along the bonded circuitry to the system bonding jumper. Since this impedance path is supposed to be very low, it will trip the overload protection (V/R=I). However, with 480 VAC, you better be careful of just how low the impedance is! AEMC sells a clamp-on device that can measure this. At 480 VAC, with a 10 amp circuit breaker, you'd need a maximum of 480 VAC / 10A = 48 ohms of resistance. If the circuit breaker is 200 amps, then the resistance requirement drops down to 480 / 200 = 2.4 OHMS! Since it's not even possible to get a circuit breaker to trip at 1000 A (480 / 1000 = 0.5 amps or less), that's why the equipment ground provisions are in the NEC. Instead, you need an entirely different type of grounding system. Your choices are either ungrounded deltas or a high resistance ground. With an "ungrounded" system, the transformer is delta-delta. There is no physical connection to ground at all. If one of the phase legs becomes grounded, the first ground fault is "free", no harm done. You can continue to operate in spite of a ground fault. The second one is free only if it doesn't happen on another phase. Otherwise, both ground faults become energized and you go through a "burn down" because you've got an effective phase-to-phase fault (very nasty). Detecting a ground fault is very easy (string a light between each phase and ground), but finding it is very difficult. You only know that you have a ground fault, not the location. Also the temptation is to keep running and not to track it down, very, very bad. You also have very nasty transients. Since everything is effectively capacitively grounded (two conductors with an air gap forms a capacitor), every time you have a transient, it gets amplified because the "capacitor" is draining. Ungrounded deltas tend to shred insulation and cause all kinds of "electrical erosion" problems over time. Overall, I just can't say enough bad things about ungrounded deltas. The "downtime saving" that they achieve in the short term is completely overshadowed by all the long term maintenance problems that they cause. The alternative is a high resistance ground. This is the best ground system that money can buy. It is even better than a solidly grounded system even at 480 VAC. You use a delta-wye transformer but the neutral leg is tied to ground via a grounding resistor (usually 15-25 ohms). Measuring (and tripping) ground fault current is very easy, and it can operate similar to an ungrounded delta for a short period of time. By employing several ground fault relays with graded time-current curves, you can achieve selective coordination (shut down only the faulty equipment). Since the system capacitance is continuously drained to ground, you don't have the nasty transient problems of an ungrounded delta either. In effect, they have the selective coordination of a solidly grounded wye, the lowest transients of any of the three systems, and no nuisance tripping. The only downside is that they are somewhat more expensive than solidly grounded wyes. That being said, my current employer uses them on ALL 480 VAC and higher systems. High resistance grounds are the most common system in medium and high voltage systems. They are definitely not subject to nuisance trips since you usually set the ground fault relay to say 10-25 amps. IEEE also recommends this when it comes to the high current situation that the NEC codes allude to. If you want more details and you'd like to purchase relatively inexpensive ground fault relays, I've found Bender to be an extremely reputable and reliable source of information. About the only "problem" with Bender is that they have a very active engineering group which means they are constantly coming out with new models, customer specific models, and other tweaks and improvements. They are very experienced with ground fault monitoring for underground coal mines (their offices are in the Pennsylvania coal field area). Talking to their sales reps and applications engineers is very beneficial. The "ground check" system is something entirely related to MSHA. Achieving good grounding and bonding in a portable cable mining environment is a challenge. There are several variations on it but essentially the system adds additional conductors that are used to actually run low current/voltage monitoring signals across the bonding circuits on mining equipment and cables. If the continuous bonding is broken, the ground check system trips the circuit breakers and shuts down the offending equipment. About the only positive advantage of this system is that you can unplug "hot" medium and 480 VAC plugs while they are "live'. The ground check pins (which are shorter than the power conductors) will trip the circuit breaker and safely de-energize the equipment if someone "pulls the plug". Energizing is also extremely safe since once the plug is inserted, the ground check relay has to be reset to energize the connection. It makes coupling and uncoupling medium voltage equipment idiot resistant (can't say idiot proof...idiots constantly come up with ingenious ways to kill themselves). These two PDF's explain both systems: http://www.msha.gov/S&HINFO/TECHRPT/GROUND/groundck.pdf http://www.kilowattclassroom.com/Archive/NeutRes.pdf

[TimWilborne 108]

I'm still digesting your post Paul...didn't want you to think it had fallen upon deaf ears

[TConnolly 13]

Tim, ABB makes an earth leakage monitoring relay for online monitoring of circuit to ground resistance. I'm not sure of the ABB number, Entrelec used to make them but they have been swallowed by ABB. They make them for both AC and DC motors. Check it out, it might be what you are looking for.

[paulengr 44]

Two more items that you may want to consider if you ever get into the situation that are in common use with utilities: differential relays, and distance relays. This is getting way out astray from the original topic (ground fault relays) but it is highly relevant because when you look at this problem from a utility point of view, there are alternatives that are more sensitive and can be less expensive and work well even with capacitively grounded (err, ungrounded), and solidly grounded systems. But except in rare circumstances, you will hardly ever encounter them in an industrial power or controls system. A distance relay is fairly well explained in one of the more popular distance rlay books (the GE Multilin version): pm.geindustrial.com/FAQ/Documents/Alps/GER-3966.pdf Distance relays rely on looking for large changes in the impedance characteristics of a power transmission line. Whenever you have a large change in impedance such as a tree falling on a power line, these relays detect the change and trigger some action (such as opening a circuit breaker). They are used over long distances, and their specs essentially relate back to the distance over which they can operate. They are immune to the type of grounding system and associated impedances. The second option, a differential relay is relatively simple in concept, but very complicated and expensive in practice. This is a type of "unit protection" relay, meant to protect a piece of equipment rather than a whole system like the distance relay. Conceptually, place two power meters on both sides of a device (the "unit"), such as a transformer, and measure the difference between them. This would be extremely simple to build except that you really need to look at power (voltage + current, in all 3 phases) in vector form (to take out any phase differences), leading to in simple cases 6 CT's (if there is no voltage/current transformation) or in more complex ones, 6 CT's and 6 PT's. It's the measurement transformers that make the system very expensive, but when that transformer costs you several hundred thousand dollars, a few extra measurement relays isn't a lot of money...I think my "back of napkin" number usually runs $25K-$30K for these, PLUS labor (double or triple it depending on whether the equipment is metalclad or overhead design). Differential relays work regardless of the type of grounding system (even ungrounded). I love the concept but given that in practice it gives you almost no "early warning" protection on a transformer and does so little compared to the relatively low cost of a high resistance ground fault system, that I've never had the opportunity to do anything more than look at the idea and reject it for cost reasons. If I was on the other side of the fence working for a utility where business inefficiency is valuable (higher costs = higher profits) as long as it can somehow be described in terms of some very nebulous concept of "reliability", then I'd of course favor putting in differential relays on everything from transformers to coffee pots. Although digital differential relays with radio, carrier current, or fiber data transmission systems have been developed to allow differential relays to operate over very long distances, distance relays are still simpler, less expensive, and can operate at a point rather than with two separate sets of measurement transformers. In practice I've never heard of even a utility using a differential relay instead of a distance relay.

[TimWilborne 108]

I'm taking all of this great information and have come up with several options to present to the safety director Thanks everyone! TW