Instrument Transformer Test Switches

Connections made between instrument transformers and receiving instruments such as panel- mounted meters and relays must be occasionally broken in order to perform tests and other maintenance functions.

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An accessory often seen in power instrument panels is a test switch bank, consisting of a series of knife switches. A photograph of a test switch bank manufactured by ABB is seen here:

Some of these knife switches serve to disconnect potential transformers (PTs) from receiving instruments mounted on this relay panel, while other knife switches in the same bank serve to disconnect current transformers (CTs) from receiving instruments mounted on the same panel.

For added security, covers may be installed on the switch bank to prevent accidental operation or electrical contact. Some test switch covers are even lock-able by padlock, for an added measure of access prevention.

Test switches used to disconnect potential transformers (PTs) from voltage-sensing instruments are nothing more than simple single-pole, single-throw (SPST) knife switches, as shown in this diagram:

There is no danger in open-circuiting a potential transformer circuit, and so nothing special is needed to disconnect a PT from a receiving instrument.

A series of photographs showing the operation of one of these knife switches appears here, from closed (in-service) on the left to open (disconnected) on the right:

Test switches used to disconnect current transformers (CTs) from current-sensing instruments, however, must be specially designed to avoid opening the CT circuit when disconnecting, due to the high-voltage danger posed by open-circuited CT secondary windings. Thus, CT test switches are designed to place a short-circuit across the CT’s output before opening the connection to the current-measuring device. This is done through the use of a special make-before-break knife switch:

A series of photographs showing the operation of a make-before-break knife switch appears here, from closed (in-service) on the left to shorted (disconnected) on the right:

The shorting action takes place at a spring-steel leaf contacting the moving knife blade at a cam cut near the hinge. Note how the leaf is contacting the cam of the knife in the right-hand and middle photographs, but not in the left-hand photograph. This metal leaf joins with the base of the knife switch adjacent to the right (the other pole of the CT circuit), forming the short-circuit between CT terminals necessary to prevent arcing when the knife switch opens the circuit to the receiving instrument.

A step-by-step sequence of illustrations shows how this shorting spring works to prevent the CT circuit from opening when the first switch is opened:

It is typical that the non-shorting switch in a CT test switch pair be equipped with a “test jack” allowing the insertion of an additional ammeter in the circuit for measurement of the CT’s signal. This test jack consists of a pair of spring-steel leafs contacting each other in the middle of the knife switch’s span. When that knife switch is in the open position, the metal leafs continue to provide continuity past the open knife switch. However, when a special ammeter adapter plug is forced between the leafs, spreading them apart, the circuit breaks and the current must flow through the two prongs of the test plug (and to the test ammeter connected to that plug).

A step-by-step sequence of illustrations shows how a test jack maintains continuity across an opened knife switch, and then allows the insertion of a test probe and ammeter, without ever breaking the CT circuit:

When using a CT test probe like this, one must be sure to thoroughly test the electrical continuity of the ammeter and test leads before inserting the probe into the test jacks. If there happens to be an “open” fault anywhere in the ammeter/lead circuit, a dangerous arc will develop at the point of that “open” the moment the test probe forces the metal leafs of the test jack apart! Always remember that a live CT is dangerous when open-circuited, and so your personal safety depends on always maintaining electrical continuity in the CT circuit.

This close-up photograph shows a closed CT test switch equipped with a test jack, the jack’s spring leafs visible as a pair of “hoop” shaped structures flanking the blade of the middle knife switch:

In addition to (or sometimes in lieu of ) test switches, current transformer secondary wiring often passes through special “shorting” terminal blocks. These special terminal blocks have a metal “shorting bar” running down their center, through which screws may be inserted to engage with wired terminals below. Any terminals made common to this metal bar will necessarily be equipotential to each other. One screw is always inserted into the bar tapping into the earth ground terminal on the terminal block, thus making the entire bar grounded. Additional screws inserted into this bar force CT secondary wires to ground potential. A photo of such a shorting terminal block is shown here, with five conductors from a multi-ratio (multi-tap) current transformer labeled 7X1 through 7X5 connecting to the terminal block from below:

This shorting terminal block has three screws inserted into the shorting bar: one bonding the bar to the ground (“G”) terminal on the far-left side, another one connecting to the “7X5” CT wire, and the last one connecting to the “7X1” CT wire. While the first screw establishes earth ground potential along the shorting bar, the next two screws form a short circuit between the outer two conductors of the multi-ratio current transformer. Note the green “jumper” wires attached to the top side of this terminal block shorting 7X1 to 7X5 to ground, as an additional measure of safety for this particular CT which is currently unused and not connected to any measuring instrument.
The following illustrations show combinations of screw terminal positions used to selectively ground different conductors on a multi-ratio current transformer. The first of these illustrations show the condition represented in the previous photograph, with the entire CT shorted and grounded:

This next illustration shows how the CT would be used in its full capacity, with X1 and X5 connecting to the panel instrument and (only) X5 grounded for safety:

This final illustration shows how the CT would be used in reduced capacity, with X2 and X3 connecting to the panel instrument and (only) X3 grounded for safety:

Article from Lessons In Industrial Instrumentation by Tony R. Kuphaldt – under the terms and conditions of the Creative Commons Attribution 4.0 International Public License

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