Energy Measurement

Question 1

What kind of service requires instrument transformers?

Answer 1

One over 200A and/or 600V.

Question 2

What is a “self contained meter”?

Answer 2

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

Question 3

What are the three common single phase services?

Answer 3

120/240 V 3 wire 1-Φ.

120 V 1-Φ.

120/208 3 wire Network 2-Φ.

Question 4

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

Answer 4

A 120/240 V meter socket.

Question 5

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

Answer 5

A billboard sign.

Yes, 30 A.

Question 6

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

Answer 6

Small city center parking lots,

Single dwellings in an area supplied by a network.

Question 7

Which service requires a 5 jaw meter socket?

Answer 7

120/208 V Network service;

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

Question 8

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

Answer 8

The common conductor;

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

Question 9

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

Answer 9

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

Question 10

What is a tickler?

Why is it used?

What happens if it is removed?

Answer 10

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.

Question 11

What services can three phase meter sockets be used for?

Answer 11

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]

Question 12

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

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

Answer 12

The right side, same as the tickler;

7 jaws.

Question 13

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

Answer 13

13 jaws.

Question 14

What benefit do instrument transformers provide?

Answer 14

Isolation from high voltage and currents.

Question 15

What are the two basic types of instrument transformers?

When used for metering, what is their mounted polarity?

Answer 15

Potential transformers and current transformers;

Subtractive.

Question 16

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

Answer 16

120 V.

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

Question 17

Which type of instrement transformer must have a fused primary?

Answer 17

Potential transformers.

Question 18

Is the secondary of an instrument transformer always grounded?

Answer 18

Yes, for safety.

Question 19

What is the terminal H₁ generally identified by?

Answer 19

A white dot.

Question 20

How is a current transformer connected?

Answer 20

In series with the load.

Question 21

A CT is like a step - _________ transformer.

Answer 21

Step-Up.

Question 22

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

Answer 22

5 A.

Question 23

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

Answer 23

Measuring current or power within a process;

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

Question 24

What are the three types of CT?

Answer 24

Wound primary;

Bar type;

Toroid type.

Question 25

Why are wound primary CTs ideal for Metering?

Answer 25

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

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

Question 26

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

Answer 26

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

Question 27

What are two other names for a Toroid type CT?

Answer 27

Doughnut or window.

Question 28

What is a benefit of Toroid type CTs?

What is a drawback?

Answer 28

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

The conductors must be disconnected to be removed.

Question 29

Why must a CT secondary always be close circuited?

Answer 29

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.

Question 30

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

Answer 30

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

Question 31

What is Burden?

What is it rated in?

Answer 31

The load attached to a CT secondary.

(Ohms)Ω.

Question 32

What could make CT measurements innacurate?

How do we prevent this?

Answer 32

10 AWG conductors and no more than 8 m long help limit circuit impedance.

Overburden;

Question 33

Which equipment will an electrician install, in regards to service and metering?

Answer 33

Install Service box;

Install CT and PT cabinet;

Install all piping;

Provide kW/h meter;

Mount PTs and CTs (supply Auth provide);

Connect CT primaries.

Question 34

Why would a transfer switch be used for metering?

Answer 34

To allow a single meter to measure different values.

Question 35

A common voltmeter transfer switch has how many positions?

What design feature does it use, while switching?

Answer 35

8 positions.

Break before make.

Question 36

What are some other names for a “truth table”?

What does it mean when an x is in the box?

Answer 36

Target table;

Sequence chart.

Electrical continuity.

Question 37

How many voltages does a typical voltmeter transfer switch measure?

Answer 37

6:

V_AB; V_BC; V_CA; V_AN; V_BN; V_CN;

There are also two off positions.

Question 38

An ammeter transfer switch must always have which design feature?

Answer 38

Make before break, or overlapping action.