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Electrical Lab Transformers Period 3 SAIT-Alberta > Energy Measurement > Flashcards

Flashcards in Energy Measurement Deck (38)
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What kind of service requires instrument transformers?

One over 200A and/or 600V.


What is a "self contained meter"?

Meters which measure the actual current and voltage and do not use transformers.


What are the three common single phase services?

120/240 V 3 wire 1-Φ.

120 V 1-Φ.

120/208 3 wire Network 2-Φ.


Which meter socket may be used for a 240/480 V service connection?

A 120/240 V meter socket.



What is an example of 120 V single phase service? Is there generally an ampacity limit?

A billboard sign.

Yes, 30 A.


Where could a 120/208 V Network service be found?

Small city center parking lots,

Single dwellings in an area supplied by a network.


Which service requires a 5 jaw meter socket?

120/208 V Network service;

{A 240 V 4 wire ▲ service (on the 3 Φ 3 wire connection)}may use one.


What is the "neutral" referred to in a 120/208 Network service?

The common conductor;

It can carry as much current as one of the phase conductors.


What care must be taken on a 120/208 V network service?

Devices should be rated for dual voltage (208/240 V) or 208 V.


What is a tickler?

Why is it used?

What happens if it is removed?

A conductor which connects the common conductor to the fifth jaw;

It maintains proper voltage drops and phase relationships;

The voltage drop is 104 V instead of 120, because the coils become ungrounded.


What services can three phase meter sockets be used for?

3-Φ 4 wire wye;

[120/208 V], or [277/480 V], or [347/600 V];

3-Φ 4 wire delta.

[240 V 3-Φ], and [120/240 V 1-Φ]


Which side of the meter socket will the high leg go?

How many jaws does the 3-Φ meter socket have?

The right side, same as the tickler;

7 jaws.


How many jaws are on a meter socket which accounts for data communication?

13 jaws.


What benefit do instrument transformers provide?

Isolation from high voltage and currents.


What are the two basic types of instrument transformers?

When used for metering, what is their mounted polarity?

Potential transformers and current transformers;



When using a potential transformer for normal revenue metering, what is the volt meter typically rated for?

120 V.

(This is generally lower than the circuit voltage and saves us money).


Which type of instrement transformer must have a fused primary?

Potential transformers.


Is the secondary of an instrument transformer always grounded?

Yes, for safety.


What is the terminal H1 generally identified by?

A white dot.


How is a current transformer connected?

In series with the load.


A CT is like a step - _________ transformer.



What is the typical rated current of a CT (used for revenue metering) secondary?

5 A.


Besides metering services over 200 A or 600V, what other uses does a current transformer typically have?

Measuring current or power within a process;

Operation of: Overload relays, overcurrent devices and ground fault protection relays..


What are the three types of CT?

Wound primary;

Bar type;

Toroid type.


Why are wound primary CTs ideal for Metering?

They allow control of the turn ratio, which enhances accuracy.

They are not useful for circuit protective devices, because they saturate easily.


Why are bar type CTs very suitable for circuit protection applications?

Because they are able to handle the high mechanical stress brought on by fault current.


What are two other names for a Toroid type CT?

Doughnut or window.


What is a benefit of Toroid type CTs?

What is a drawback?

The ratio can be manipulated by repeatedly passing the conductor through the center;

The conductors must be disconnected to be removed.


Why must a CT secondary always be close circuited?

Because incredibly high voltages may be present across the open terminals;

This is why it is never fused, and shorted before a meter is disconnected.


Why does no short circuit occur on a CT secondary, when a shorting bar is used?

The counter emf (Lenz's Law) limits the current on the secondary winding.