1. Suggestion on How to Use
Industry Trainers are encouraged to use this material in their sessions
Download the presentation file
Print the Notes pages and read them as you view the presentation in the “Slide Show” view. In this way you see the slides in large format and have animation (when available)
Suggestion on How to Use
Download the presentation file. Print the Notes pages and read them as you view the presentation in the “Slide Show” view. In this way you see the slides in large format and have animation (when available).

2. Voltage Rating

3. Voltage Rating
Voltage rating of the OCPD must be equal to or greater than the system voltage.
Voltage rating determines the ability of the OCPD extinguish the arc during an overcurrent condition.
When applying overcurrent protective devices, the voltage rating of the OCPD must be equal to or greater than the system voltage. The voltage rating determines the ability of the OCPD to extinguish the arc during an overcurrent condition. For instance, if the OCPD has a voltage rating of 250V and it was mistakenly applied in a 600V application, the OCPD may not be able to extinguish the arc during an overcurrent condition and could rupture.

4. Voltage Rating
NEC (A) permits 300V cartridge fuses to be used on circuits not exceeding 300V between conductors or single-phase line-to- neutral loads not exceeding 300V.
NEC (B) requires fuseholders to reject fuses with lower voltage ratings.
NEC (C) requires the voltage rating to be marked on the fuse barrel or label.
NEC allows fuses rated 600V or less to be used for voltages below their rating.
NEC (A) permits 300V cartridge fuses to be used on circuits not exceeding 300V between conductors or single-phase line-to-neutral lighting circuits not exceeding 300V. Thus, a 300V cartridge fuse could be applied in a 240V, three-phase circuit or in a 277V, single-phase lighting circuit. NEC (B) requires fuseholders to reject fuses with lower voltage ratings. Thus, a 600V fuseholder would not be designed to accept a 250V fuse. NEC (C) requires the voltage rating to be marked on the fuse barrel or label as shown in the picture of the LPJ fuse.
NEC allows fuses rated 600V or less to be used for voltages below their rating. Thus, a 600V fuse could be applied in a 240V system, provided the fuse was installed in a 600V rated fuseholder or disconnect.

5. Voltage Rating
NEC (E) requires circuit breakers to be marked with a voltage rating that indicates the circuit breaker’s capability to interrupt overcurrents between phases or phase to ground.
Voltage rating can be “straight” or “slash” rated.
NEC (E) requires circuit breakers to be marked with a voltage rating that indicates the circuit breaker’s capability to interrupt overcurrents between phases or phase to ground. The voltage rating for a circuit breaker can be either “straight” or “slash” rated.

6. Voltage Rating
NEC covers the requirements for straight rated and slash rated circuit breakers.
A straight rated CB (480V) can be applied on systems that no more than 480 volts between any two conductors.
A two-pole circuit breaker applied on a 3-phase, corner-grounded delta system must be marked 1Φ- 3Φ.
NEC covers the requirements for straight rated and slash rated CBs. A straight rated CB (such as 480V) can be applied on systems that have no more than 480V between any two conductors. A two-pole circuit breaker applied on a 3-phase, corner-grounded delta system must be marked 1Φ-3Φ. This marking indicates the circuit breaker has been tested and is suitable for this type of system.

7. Voltage Rating NEC 240.85 (Cont)
A slash rated circuit breaker, such as 480Y/277V, can only be applied in a solidly grounded circuit where the nominal phase to ground voltage does not exceed the lower number (277V) and the phase to phase voltage does not exceed the larger number (480V).
NEC requires that a circuit breaker with a slash rating, such as 480Y/277V, can only be applied in a solidly grounded circuit where the nominal phase to ground voltage does not exceed 277V and the phase to phase voltage does not exceed 480V. The underlined words, “solidly grounded”, indicates the change from the 1999 to 2002 NEC Code. This was needed to emphasize that slash-rated devices were not appropriate on resistance-grounded, ungrounded, and corner grounded systems.

8. This picture shows an example of a 480/277V slash-rated circuit breaker.
As discussed previously, a slash rated circuit breaker is one with two voltage ratings separated by a slash, such as 480Y/277 volt as shown in the picture.

9. Voltage Rating 3-Pole, 480/277V CB Three Phase C
Corner Grounded Delta system C B A
Slash rated circuit breakers, applied in solidly grounded circuits, must be applied properly to assure the ratings are not exceeded. For example, if a 480/277V circuit breaker was misapplied in a 480V corner-delta system, a situation, such as the mishap of a slipped screwdriver causing one phase to short to ground would create a single-pole short-circuit that exceeds the voltage rating of the circuit breaker. It would create a 480 volt arc across only one pole of the circuit breaker, even though the maximum voltage to which one pole of the circuit breaker was tested was 277 volts. In these situations, where it is possible for full phase-to-phase voltage to appear across only one pole, a fully rated or straight rated circuit breaker must be utilized. In order to understand this concept, one must understand the type of systems on which the circuit breaker could be applied and the grounding schemes for that type of system.
Equipment Bonded to Ground

10. Slash Rated CB Application
Circuit breaker
480Y/277 slash voltage rating
480 volts
line to line A
480Y/277 Volt
Three phase
Four wire
Solidly grounded
wye system B C
277 volts
line to ground N
Now, if we have a 480Y/277V, three phase, four wire solidly grounded wye system and apply this 480/277V circuit breaker:
The smaller of the two ratings is for overcurrents at line-to-ground voltages, meant to be cleared by one pole of the device. In this example, the circuit breaker’s lower rating is 277 volts and the system line to ground voltage is 277 volts.
The larger of the two ratings is for overcurrents at line-to-line voltages, meant to be cleared by two or three poles of the circuit breaker. In this example the system’s line to line voltage is 480 volts and the circuit breaker line to line voltage rating is 480 volts. G

11. Is Slash Rated 480Y/277V Device Permissible?N
277V A B C
480V
Solidly Grounded 480Y/277V
Click on Answer
Yes OR No
Before we go on, let’s see if you understand the application of slash ratings. The system shown is a 480Y/277 volt, solidly grounded systems. Is it permissible to use 480Y/277 slash voltage rated circuit breakers on this system? Select yes or no. Just use your mouse to move the cursor to your answer and left click.

12. Is Slash Rated 480Y/277V Device Permissible?N
277V A B C
480V
Solidly Grounded 480Y/277V
Yes OR No
The correct answer is yes. A slash rated circuit breaker could be used in this system. But let’s review the reason. This is a solidly grounded 480Y/277 system, so a slash rating is permissible if two other conditions are met:

13. Is Slash Rated 480Y/277V Device Permissible?
Solidly Grounded 480Y/277V
277V
480V
277V
277V B
Yes OR No
480V
First, the lower voltage rating of the circuit breaker which is 277 volts is equal to or greater than the system line to ground voltage which is 277 volts. In this example this condition is met. C N

14. Is Slash Rated 480Y/277V Device Permissible?
Solidly Grounded 480Y/277V
277V
480V
277V
277V B
Yes OR No
480V
And second, if the larger voltage rating of the circuit breaker which is 480 volts is equal to or greater than the system line to line voltage which is 480 volts. In this example this condition is also met.
So a 480Y/277 slash rated circuit breaker complies with C N

15. Is Slash Rated 480Y/277V Device Permissible?
Corner Grounded Delta A
480V
480V
Click on Answer
Yes OR No B
480V
Let’s try another slash rating example. The system shown is a 480 volt corner-grounded, delta system. This system has the B phase grounded.
Is it permissible to use 480Y/277 slash voltage rated circuit breakers on this system? Select yes or no. Just use your mouse to move the cursor to your answer and left click. C

16. Is Slash Rated 480Y/277V Device Permissible?
Corner Grounded Delta A
480V
480V
Yes OR No B
480V
The correct answer is no. It is not permissible to use a 480Y/277 slash rated circuit breaker in this system. But let’s review the reason. This is a solidly grounded system. Let’s check the other two conditions: C

17. Is Slash Rated 480Y/277V Device Permissible?
Corner Grounded Delta A
480V
480V
Yes OR No B
480V
One condition was that, if the larger voltage rating of the circuit breaker is equal to or greater than the system line to line voltage. In this example, this condition is met since the line to line voltage is 480 volts and the larger circuit breaker rating is also 480 volts. C

18. Is Slash Rated 480Y/277V Device Permissible?
Corner Grounded Delta A
480V
480V
Yes OR No B
480V
However, the other condition is not met. Let’s see why. The lower voltage rating of the circuit breaker has to be equal to or greater than the system line to ground voltage. In this example this condition is not met. The line to ground voltage for phase A and C is 480 volts but the circuit breaker’s lower voltage rating is only 277 volts. This slash rated breaker is not properly rated for this system. C

19. Slash Rated Exercise System Voltage Secondary System Type L-L Volt L-G
Can 480Y/277
CB Be Used?
480Y/277
Solidly Grounded WYE
480
277
Yes No
480
Resistance
Grounded WYE
480
277
Yes No
480
Delta Corner Grounded
B Phase
480
480
Yes No
Before we proceed let’s test your knowledge. For each of the four separate examples shown answer yes or no as to whether a 480Y/277 volt circuit breaker can be used in compliance with The first is a 480Y/277 volt, solidly grounded system with 480 volts line to line and 277 volts line to ground. Can a 480Y/277 volt circuit breaker be used?
Using your mouse, click on your answer either yes or no.
480
Delta
Ungrounded
480 *
Yes No
* Ungrounded delta systems - phase conductors are capacitively coupled to ground

20. Slash Rated Exercise System Voltage Secondary System Type L-L Volt L-G
Can 480Y/277
CB Be Used?
480Y/277
Solidly Grounded WYE
480
277
Yes No
480
Resistance
Grounded WYE
480
277
Yes No
480
Delta Corner Grounded
B Phase
480
480
Yes No
Let’s check your response. The first is a 480Y/277 volt, solidly grounded system with 480 volts line to line and 277 volts line to ground.
Can a 480Y/277 volt circuit breaker be used? The answer is Yes, since it is a solidly grounded system, the lower device rating of 277 volts equals the system line to ground voltage of 277 and the higher device rating of 480 equals the line to line system voltage of 480 volts.
Proceed and do the next one; the resistance grounded wye.
click on your answer.
480
Delta
Ungrounded
480 *
Yes No
* Ungrounded delta systems - phase conductors are capacitively coupled to ground

21. Slash Rated Exercise System Voltage Secondary System Type L-L Volt L-G
Can 480Y/277
CB Be Used?
480Y/277
Solidly Grounded WYE
480
277
Yes No
480
Resistance
Grounded WYE
480
277
Yes No
480
Delta Corner Grounded
B Phase
480
480
Yes No
Let’s check your second response. The answer is No, because it is a resistance grounded system which is not defined as a “solidly grounded system”. A slash rated device does not comply. Proceed and do the third one; delta corner grounded. Click on your answer.
480
Delta
Ungrounded
480 *
Yes No
* Ungrounded delta systems - phase conductors are capacitively coupled to ground

22. Slash Rated Exercise System Voltage Secondary System Type L-L Volt L-G
Can 480Y/277
CB Be Used?
480Y/277
Solidly Grounded WYE
480
277
Yes No
480
Resistance
Grounded WYE
480
277
Yes No
480
Delta Corner Grounded
B Phase
480
480
Yes No
Let’s check your third response. The answer is No. This is a delta corner grounded system. Even though it is solidly grounded, the lower device rating of 277 volts is less than the system line to ground voltage of 480 volts. So a slash rated device does not comply.
Proceed and do the fourth one; delta ungrounded. Click on your answer.
480
Delta
Ungrounded
480 *
Yes No
* Ungrounded delta systems - phase conductors are capacitively coupled to ground

23. Slash Rated Exercise System Voltage Secondary System Type L-L Volt L-G
Can 480Y/277
CB Be Used?
480Y/277
Solidly Grounded WYE
480
277
Yes No
480
Resistance
Grounded WYE
480
277
Yes No
480
Delta Corner Grounded
B Phase
480
480
Yes No
Let’s check your fourth response. The answer is No. This is not a solidly grounded system so a slash rated device does not comply.
480
Delta
Ungrounded
480 *
Yes No
* Ungrounded delta systems - phase conductors are capacitively coupled to ground