Grounding Transformer Output

  • Thread starter Bob Dannenfelser
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Thread Starter

Bob Dannenfelser

A question has come up as to the necessity of grounding the output side of a voltage transformer. The model in question is a Square D Industrial Control Transformer. Input voltage is 220vac, output is 120vac. If it is customary to ground one side (neutral?) of the output side, how do you know which side is Line and which is Neutral? Instructions on the unit do not mention the need to do this. Thanks for your assistance. Bob Dannenfelser Mottley Air Power Baltimore, MD [email protected]
 
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Steve Myres, PE

"Neutral" in this usage is synonymous with "groundED conductor" (NOT the same as "groundING conductor"). Whichever of the output terminals you bond to ground becomes the neutral by virtue of being at ground potential. Although ungrounded control systems are sometimes done, I would recommend grounding to avoid the system floating. Steve Myres, PE [email protected]
 
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Peter Veselosvky, konnexis

Typically an isolated transformer can be grounded on either of the two wires of the secondary winding. The grounded wire becomes the neutral, the ungrounded will be the line. If a transformer remains unground, and if it is beyond certain rating (which quite low), all equipment connected to it must be doubly insulated to meet electrical code. For the secondary winding to remain ungrounded, a high impedance resistor should be connected to ground in order to prevent high potential difference between the secondary winding and the ground. A high potential difference will cause dialectric breakdown and may result in a permanent equipment damage. Peter Veselovsky [email protected]
 
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Crucius, Wesley

It is most likely an Isolation Xfmr, and as such you can theoretically ground either side, and you should. This is usually referred to a bonding. If you have an Auto Xfmr, this is essentially already done for you, internally.
 
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Richard King

Bob, At the alumina plant I am currently working for, the 117VAC Instrument supply has a centre tap that is grounded, so each active is approx 56VAC to ground. If you don't have a centre tap, you can still have an isolated 120VAC supply. If you still intend to ground one of the 120VAC legs, make sure there is no internal centre tap. Whether it requires grounding or not would depend on your countries electrical regulations. regards, Richard Alesa Alusuisse Engineering Australia Pty Limited. Phone: +61 7 3218 3555 Fax : +61 7 3236 0155 Email: [email protected] Web : http://www.alesa.com.au
 
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Michael Griffin

120VAC control circuits are typically grounded, but there are some applications which use ungrounded systems. The neutral side is grounded, but as to which side of the circuit should be the neutral depends upon the design of the circuit. I suggest that you contact an experienced industrial electrician to connect this transformer for you. There are undoubtedly a number of matters in addition to grounding which will need to be looked into, and if you are not familiar with them you won't even know what questions to ask. ********************** Michael Griffin London, Ont. Canada [email protected] **********************
 
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Typicaly the "X2" lead on a single phase secondary is tied to ground, and used as the neutral lead. I don't know if there is any reason (except perhaps tradition), although, if there are multiple transformers involved, it may be necessary so phasing doesn't become an issue. Also, certain types of control systems (for instance, automatically switched power factor correction capacitor banks) often require proper phase relation between the primary three phase line, and the 120V secondary .... in these cases it DOES make a difference which side is considered neutral, and grounded. Unless the transformer secondary neutral needs to float above ground I prefer to ground it at the transformer (and bond this point to the enclosure central ground point). This promotes safety (the neutral sits a 0V), and makes rough troubleshooting easier (it's usually more convenient to get at a ground point, and probe AC voltages, than to find a neutral terminal).
 
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Bob Peterson

Traditionally, the "X2" leg is grounded for grounded control circuits and the ungrounded leg is fused. I do not believe this is a requirement, but is the common practice, it will no doubt work as well grounding the X1 leg. However, its not unheard of for control circuits to be ungrounded (i.e.-no neutral). In that case neither leg is grounded and both are fused (or otherwise protected). Bob Peterson
 
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Hi, Just a word of warning: in my experience, leaving the secondary ungrounded can cause common-mode problems. I had an application on a conveyor line where the I/O was spread out over a long distance. The I/O circuits were run from 120V stepdown transformers which were connected on legs of different 480V circuits, and the secondaries (120V) were left floating. We had problems strange problems with latchup and false inputs. We disconnected the secondary load of one of the transformers and then measured the different voltages. Between any leg and ground, the voltage was less than 120V; however, between the two floating secondaries, the voltage varied between 200 - 350V! I think that in this case we were seeing a capacitively-coupled common-mode problem; anyway, grounding both secondaries got rid of it. Also, in another post someone mentioned autotransformers. In this case, the common leg of the autotransformer should be connected to neutral, whether it is connected as a step-up or step-down device. Regards Willy Smith Numatics, Inc. Costa Rica
 
If your system is an earthed system, then you have to ground ANY one terminal of the output. The terminal which you connected to ground becomes neutral - As it has to mentain the same potential as the ground which is neutral- The other terminal is LINE. If you are using an un-earthed system - Some control circuitry at 110~220V uses isolated system - then you need not (shall I say should not ?) to earth the output of transformer. Arunkumar
 
I believe that grounding the transformer is good practice and required by the NEC per Article 250-20(b)(1). There is an alternative situation where "continuity of control power is required" where it may be ungrounded if you meet all the requirements of NEC Article 250-21(3). If this is OEM equipment with all circuits inside the equipment, then you should see what your listing or approval agency standards require. In regards to which lead is grounded, normally it is the "X2" lead if identified, otherwise either one should do. Bill Mostia =========================================== William(Bill) L. Mostia, Jr. PE Independent I &E Consultant WLM Engineering Co. P.O. Box 1129 Kemah, TX 77565 [email protected] 281-334-3169 These opinions are my own and are offered on the basis of Caveat Emptor.
 
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compu-weigh-jvp

In addition to the original question. I use Phase Angle Control to control the Power of the secondary of the Transformer.(75VAC) The secondary of the transformer is not used for any other purpose. To avoid getting interference in other circuits, should I ground one leg of the secondary, or leave it floating? Regards Jan van de Poll Technical Manager Compu-Weigh Pty Ltd [email protected]
 
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Jeffrey W. Eggenberger

If this is for industrial controls, then it is possible that they do not want you to ground the secondary. It is not necessary to do this. What is your application? Are you a licensed electrician qualified to be wiring control panels? I doubt it if you have to ask the question. Jeffrey W. Eggenberger Electrician: Industrial, Commercial, and Residential
 
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Jeffrey W. Eggenberger

AAAAAHHHH! I can't take it anymore! Grounding of a "Single phase" control transformer is done for safety, or in non-surge protected circuits utilizing processors or computers that are susceptible to noise on the line. The safety is provided by the fact that if the "live" portion of the circuit happens to connect to or short to the case or any exposed part of the device, then the circuit protection device (circuit breaker or fuse) will trip. Bonded lighting transformers designed for general lighting use therefore usually provide any cord and plug outlets intended for human use. During WWII it was discovered that munitions plants had a lot of down time due to shorts to ground, but that by not bonding the secondary of the transformer, production could continue un-interrupted until the short could be located. A qualified electrician can locate ground faults in a system and repair them without the resulting downtime. The control cabinets have incorporated on them ground fault detect lights that detect when there is a ground on either leg of the transformer secondary. The main 480 buss also has ground detect lights that will detect a ground on the 480 system. The reason that the neutral on your residential transformer is grounded at the panel is twofold, one is for the safety reason above, and also it prevents "floating" of the voltages on each leg. By grounding the neutral you are lowering the impedance of the supply back to the power plant. The result of grounding keeps the voltage in each leg stable and fixed. If you get a poor ground to neutral connection at your panel, or perhaps your ground rod is not doing its proper job, or your water pipe used as a grounding electrode has been compromised, then your voltages will float. This is evidenced by when the refrigerator kicks on; some of the lights in your house will get brighter, and some dimmer. Ungrounded systems are only "unsafe" if one or the other side of the secondary is grounded, and the other side becomes exposed. Industries that use ungrounded systems typically have in house electricians on hand to repair any ground faults as they occur. Also you can utilize a constant output transformer along with surge suppression in order to use PLC or computers in your control circuits. If ever you have a question about electricity or electrical controls please consult a qualified electrician. That's what we are here for! Jeffrey W. Eggenberger Electrician: Industrial, Commercial, and Residential
 
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Jeffrey W. Eggenberger

In this case they were obviously NOT using control (single phase) transformers, and yes they should have been grounded to prevent the "floating". Jeffrey W. Eggenberger Electrician: Industrial, Commercial, and Residential
 
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Bruce Axtell

When using an ungrounded control circuit, it is customary to put ground detector lights on the panel. Ground detector lights are two pilot lights wired in series across the secondary of the transformer. The center point between the two lights is then grounded. Since the lights are in series, they will be very dim, if visible at all (e.g., 120V secondary, both pilot lights should be 120V. Thus each light sees 60V.) Then if either the X1 side or the X2 side of the transformer gets grounded, one or the other lights will become full brightness, indicating a short on one or the other sides. It is also a normal practice to use push-to-test pilot lights so you can test them periodically. One advantage of ungrounded control circuits, is that a single grounding failure (either device failure or service personnel "oops") will not blow a fuse and take down the system. This could be an important consideration for down-time critical applications. Bruce Axtell Engineered Control Systems 763-421-8787 763-712-5477 fax [email protected]
 
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Michael Griffin

At 15:07 06/04/01 -0400, Sam Robinson wrote: >You don't have to be a "licensed electrician" to wire industrial control cabinets. < <clip> On the other hand, you shouldn't be wiring control circuits based only on advice you get over the internet. There have been a number of different opinions in reply to the original question, and all of them are worthless because no one has actually seen the machine the fellow wants to wire up. ********************** Michael Griffin London, Ont. Canada [email protected] **********************
 
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Jeffrey W. Eggenberger

Indeed! And I am constantly reminded of this by the quality of some of the "garage wired" panels that I am asked to install. One of these panels not only had the control transformer secondary grounded, but had the "required" ground fault light (actually two are required) that was just a bulb wired across the secondary. It was obvious that the builder had no concept of what the bulb was for, or the concept of ungrounded systems. He might also have been a "licensed electrician" who was unfamiliar with industrial controls. Jeffrey W. Eggenberger Electrician: Industrial, Commercial, and Residential
 
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Hi Sam That's an awfully sore point around here. We have the choice of risking fines or condemnation by doing it ourselves or doing it ourselves after the electricians finish doing it. It's a racket and pretty close to criminal. And yes, I do agree it has to be done safely. It's just that the state is colluding with the unions to ensure full employment for electricians. To hear the inspectors, anything that has to do with wire has to be done by not just a licensed electrician, but one with the right ratings. If we want to do it we have to hire a Master to bless and sign off on each job. I don't have a problem with code compliance at all. I do have a problem with requiring the work be done only by electricians. And to become qualified to wire what I design, I have to be an electricians flunky for thousands of hours. And the electricians have no clue on signal wiring, fieldbusses, shield bonding, transmission lines, etc. To do it entirely legally is impossible and financially unfeasable and that's stupid. regards cww
 
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