Electrical over current testing

yes.It is always better to inject primary current and test.With this test complete system with CT is tested.

I've spent years doing electrical testing and have never heard of someone using a primary injection to thump relays.

Calibration and adjustment of a protective device should only be done by a qualified person who understands the device they are servicing. I've cleaned up several messes because of this very thing. It's certainly best to use a relay test set designed specifally to initiate the test and measure time. These results need to then be verified against the manufactures data and the coordination study to ensure you are good to go.

Here's an example: http://www.atecorp.com/equipment/multiamp/epoch-i.asp

Typically the only thing you thump with primary current are circuit breakers. I had 20ka and 60ka test sets for this.

Personally I've never heard of somebody testing CTs with primary current. For acceptance tasting I used a turns-ratio and polarity tester. For typical PM service I simply use amps out of my relay set and wrap the CT an appropriate number of times to get an output level.

Switchman, listen to the engineer (ramvinod). The basic function of the relay can be tested using a current in the range of values of a CT secondary output directly to the relay, however if there are any questions or if the function of the relay has a very high importance, an "overall" test is in order. The method of wrapping primary turns through the CT, that you mentioned, is quite efficient and allows reaching currents of the needed values for a proper overall test. This proves the function not only of the relay itself, but also of the CT and the wiring loop and connections.Though you may not have heard of this previously, in the utility industry it is quite normal and is usually required by parties buying and selling large quantities of critical power.

Hi CJMcGill,

I have to admit that I was a bit surprised by your response to my answer. I also must stand by the answer that I gave the op. In part because of all the info the op left out and also in part because I still cannot fathom how one could properly test a relay utilizing a primary injection...or why.

I've tested, calibrated ,and certified literally thousands of utility grade protective relays applied in critical applications. Basler, GE, Schweitzer, and plenty of 50/51 current relays. I've done lots of startup and acceptance, both as manufacturer's engineer and as a third party independent engineer. I've done annual pm and retrofits. I've done utility, both generation and substation. I've been in countless meetings going over site specs, acceptance requirements and coordination studies. I have never had a spec, utility or otherwise, call for primary injection to test any relay. The closest I've come is the FAA having a requirement that all devices would be demonstrated at rated set points, in the case and intact, and each actual trip time recorded at the breakers. But even that was secondary 5amp tied in at the shorting blocks.

But...you know the old saying. Never say never. I would be interested though, to see a spec that included primary injection of a fault level current onto a bus to test that a relay operates a contact.

First...the way the op asked the question means he hasn't got a clue or he wouldn't have even asked it.

Second...and I say this as an experienced engineer... an answer like ramvinod's is likely to get something destroyed or someone killed. Running full load current is only 30% of the basic relays function...long-time. For example, say the op has a 3ka full load value and he runs that primary value through and trips on lont time at 100sec. He still has absolutely no idea what short-time or instantaneous values will be or if the relay will even work when subjected to these fault conditions. Is he to thump the bus with 9ka and 30ka and expect to have realistic values?

Lastly...in a forum such as this you will tend to find one sincere and intelligent question posed for every 20 vague and almost assinine queries. And even more answers of that nature. People tend to try to steer others in a direction that is safe, of sound engineering principle, and also provide some insight.

Dear Sir,

I am a manufacturer of motor control centres,LT Switch boards in India.As part of final testing of the product prior to shipment we used to inject the full load current on the swirtch board and test the relays and breakers for tripping .It is to ensure the correctness of wiring,cts,relay range,instruments etc.Only on this context I have suggested that it would be better to test the relay by injecting the primary current.

Greetings ramvinod and cjmcgill,

Please do not think of my response as harsh...that truely is not my intention. I have read many of your posts on this forum and find them to be quite excellent and knowledgable.

I worked many years for a manufacturer of LT and HT switchgear. In our test bay, as one of our final tests prior to shipping release, we also would run full load primary tests for specific times and at specific external temperatures. The test technicians would have already proved out the wiring circuits and verified all pt and ct polarities. Our primary purpose of running rated current was to ensure temperature rise and cooling was within spec.

In the field, when commissioning the switchgear, our role as engineers changes. We are there to startup the equipment safely and ensure the customer gets what he paid for. But we are also there to protect our company's backside. In this environment, changes and mistakes always carry the threat of litigation. I have ran primary injection tests in the field following pm and retrofits but this test is specific to a purpose.

I cannot count how many times I have been at a site, commissioning several million dollars worth of gear, and have been told that there is no coordination study yet and no protective device trip settings or calibrations..."just set them all up out of the way." People under pressure hate hearing NO. I would then set all my of my protection up where I could run and commission my equipment and do my job. At then, in the absense of proper paperwork, all devices were set back to minimum. At the end of the job when my technicians were complete and all acceptance papers were being signed I would bring the chief engineer in and make him witness and sign off that all devices were at minimum setting. I will admit to many times having given personnel an hour or so to jot down my settings.

CJ I appreciate the link you sent and later this evening I will fully read it. The excerpt you posted actually supports what I am saying. Don't misunderstand me...I have run many primary injection tests in the field. For the very reasons listed in your post. That is very typical and industry standard. And a heck of a lot faster.

The issue is simply this..the op asked if he had to inject primary current to test the function of his current relay. Your information even shows this as a secondary injection test...which it always is.

Again, I will read the link provided...I can never get enough info and enjoy learning new perspectives. Time to go take my 2 1/2 yr old twins to the movies for the first time. Should be a wild experience.

Switchman, first I must compliment you on the many insightful and logical answers I have seen you post, and I agree with your assessment of 1 in 20 intelligent questions on this forum. I will also acknowledge my assumption as to qualification for which I contritely apologize.

As for the question posed by the op, it clearly indicated that he was made aware of overall testing as opposed to secondary testing, though he did not use that terminology. I also agree that the danger factor of an overall test is greatly increased, and therefore should be conducted ONLY by properly trained test techs in tightly controlled environments. As has been your stated experience, the majority of testing is conducted at the secondary level. There are however standards and procedures published in several forums for overall testing, usually utilities publish their own internal versions, that detail procedures and set standards of testing and reporting the results.

As per your request I have located and will include the link to one such, of which I have copied a small portion and paste here:

Purpose

Electrical equipment such as circuit breakers, protective relays, and meters are routinely tested to verify proper operation of current sensing elements. This testing is performed using high-current, low-voltage test equipment that provides a means of adjusting the value of current and also of measuring the operating time of the device under test. The output waveform of the test current is critical and must be sinusoidal; testing with equipment that produces a non-sinusoidal waveform - such as SCR's - will not produce accurate results.

CAUTION! Current injection testing is performed on de-energized, out-of-service equipment only!

Types of High-Current Testing

· Primary Injection Testing is used to test the overall operation of a current circuit. In this type of test, a high current is injected in the Current Transformer (CT) primary winding and the resulting secondary current is measured in each of the CT secondary devices such as meters and relays. This test is primarily conducted
during commissioning of new equipment or after a major circuit modification to insure that the equipment is
correctly connected. The polarity of the current may also be critical and other equipment, such as a Phase
Angle Meter, may be used in conjunction with the high-current test source.

· Secondary Injection Testing is periodically performed on the individual devices such as relays and meters to verify the accuracy and proper operation of the equipment. These devices receive their input current from the CT secondary winding so these tests will be at a much lower level of current than that used for primary injection.

Proper operation of the current-sensing protective equipment can be verified by comparing the device operating characteristics with the manufacturers published time-current characteristic curves.

Frequency of Tests

The frequency of these preventive maintenance current tests depend up the importance of the protection: high voltage equipment will often be tested annually; medium-voltage equipment is often tested and calibrated every-other year, and a three- or four-year interval for 480 volt equipment may be considered adequate.

To accommodate you "I would be interested though, to see a spec that included primary injection of a fault level current. . ."

I must re-double the mention of the extreme safety issues of this type of test, and trust that I may have provided some insight.

Is it absolute essential to inject primary full load current to test the over current relay functioning.

No, Simulating the CT secondary current is fine, BUT if the CT is from a cheap manufacturer and the quality or accuracy ratings are suspect (or you think forged) you may need to test the CT separately to ensure it doesn't saturate, etc (but that is what you get if you don't buy quality, pre-tested products - you end up having to spend time and money testing them yourself).

If you don't have the high current generating equipment available you can test the CT during site commissioning if you wish (when large currents are readily available). It depends on the application.

nsunni, please forgive the depth of our details. I see clearly now that all posts have been in sync, and we (at least I) got carried away in details.

The short answer is that most tests or calibrations will not be made at full primary amps, but at about 5 amps injected into the CT secondary terminals at the test switch.

Full primary amp tests will usually be used on special occasion, such as when first built or installed, or if a question arises that can only be answered by a full amp load injection in controlled settings, or by contractual obligation as required.

Since we have not been informed of your situation, this is the best we can do.

PS, refreshing discussion folks, & enjoy the movie.