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Audel electrical course for apprentices and journeymen - Rosenberg P.

Rosenberg P. Audel electrical course for apprentices and journeymen - Wiley & sons , 2004. - 424 p.
ISBN: 0-764-54200-1
Download (direct link): audelelectricalcourseforapprentices2004.pdf
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Opening the Secondary Circuit
The secondary circuit of a CT must never be opened when under load. Current transformers are often designed so that the cover over the secondary terminals shorts the secondary out if the cover is removed.
310 Chapter 29
There are two reasons for never opening the secondary under load:
1. A dangerously high voltage may be developed that may cause injury, even if not fatal, should anyone come in contact with the secondary circuit.
2. The high flux resulting from the open circuiting of the secondary may saturate the iron to such a degree as to seriously impair the ratio of the transformer, making it unfit for further use where accurate power measurements are required.
Consider the opening of the secondary of a CT under load conditions by comparing it with a conventional potential transformer. If the secondary load of a conventional potential transformer is removed, the current decreases in the primary until only the exciting current remains. In a current transformer the primary current is always the current in the line conductor, so when the secondary of a CT is opened, all of the primary current becomes exciting current. The flux and the induced voltage in the primary may rise to a high value. This abnormal flux will raise the secondary voltage to a very high value, depending upon the amount of current in the line conductor. It may puncture the secondary insulation or produce an arc at the point where the secondary circuit has been opened.
Symbols
The usual symbols for potential transformers are shown in Figure 29-3. The symbols for current transformers are shown in Figure 29-4.
tmoJ
|Tnnr|
PRIMARY
u
PRIMARY
SECONDARY
Figure 29-3 Potential transformer symbols.
Figure 29-4 Current transformer symbols.
Ground Fault Protection
Current transformers are used for ground fault interrupters (GFIs) and ground fault circuit interrupters (GFCIs). These two are frequently referred to as zero-sequence transformers.
In the chapter on electromagnetism, it was shown that if all conductors of a circuit were kept together, the magnetic flux encircling the conductors cancelled out. This is the principle of the operation of ground fault circuit interrupters.
Instrument Transformers 311
Figure 29-5 illustrates the principle involved. The current transformer, CT, encircles all of the conductors, A, B, C, and N. When there is no ground fault on the load side of the transformer, the flux cancels out so no current flows from the secondary through the relay coil.
„G! R£LAY TO TRIP
' MECHANISM
Figure 29-5 Principle of a ground fault interrupter.
Now a ground (G,) occurs on phase C, and a part of the current in C returns through the equipment-grounding conductor or the earth, by way of the dashed line. Thus, there occurs an unbalance in the circuits through the CT coil, causing a flux that induces a voltage into the secondary, S. Current flows through the relay, energizing it and closing the contacts, which causes the circuit to be tripped off.
The NEC requires GFIs on high-capacity services to prevent burn-downs of equipment due to faults.
Figure 29-5 is the simplest form. There are many variations and uses. One is to sound an alarm to indicate that one phase is grounded. Note resistor R in the neutral ground. This resistor type of neutral grounding prevents shutdowns when a ground occurs. If two phases ground, then the overcurrent devices will trip or blow.
Another such device is the GFCI, which may be called the people protector. It is called for on receptacle circuits and other circuits in many places in the NEC. Figure 29-6 shows a GFCI.
The GFCI is set to trip at 0.005 ampere. If the hot conductor goes to the case of a saw or other equipment and the grounding conductor is intact and properly connected, the fuse should blow or the breaker trip. If one were holding the saw, for example, and
312 Chapter 29
r\
T
LOAD
TO TRIP MECHANISM
Figure 29-6 Ground fault circuit interrupter.
standing on the ground, in all probability one wouldn’t get hurt. If the ground were of high resistance, however, the breaker might not trip and the person might be killed.
With the GFCI in the circuit, as soon as 5 milliamperes passed through the person to ground, the secondary would open instantly. The person would receive a shock but would live to tell about it.
GFCIs are available in separate portable equipment, receptacles, both plug-in and permanent types, and also in circuit breakers for panel installation.
Questions
1. Why are instrument transformers used? Explain fully.
2. What is the most common maximum secondary current of a current transformer?
3. What is the most common secondary voltage of a potential transformer?
4. Explain how the constant is figured for a current transformer.
5. Explain how the constant is figured for a potential transformer.
6. Give the symbols for instrument transformers and label them.
7. Should the secondary of a current transformer ever be opened under load? Explain.
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