Overcurrent Protection – Part Three


In Article 
Overcurrent Protection – Part One ",  which was an Introduction to Overcurrent Protection, I provide the basic information needed for best understanding of the Overcurrent protection.

Also, In Article " Overcurrent Protection – Part Two ", I answered the following questions:

  • How to size the overcurrent protection devices (OCPDs)? 
  • How to select the proper overcurrent protection for certain applications? 

Today, I will answer the following questions: 

  • How to select the proper overcurrent protection devices (OCPDs) for certain application? 



Selecting the proper overcurrent protection devices (OCPDs) for certain application




There are several types of overcurrent protection devices (OCPDs) that can be used to protect equipment and people from overcurrents such as fuses, circuit breakers and Thermal overload relays.



1- overcurrent protection devices (OCPDs) Ratings:





Breaker or Fuse Type
Conditions
Rating
Fuses and Fixed-Trip Circuit Breakers**

The standard ampere ratings for fuses and inverse time circuit breakers shall be considered as follows:

15, 20, 25, 30, 35, 40, 45, 50, 60, 70, 80, 90, 100, 110, 125, 150, 175, 200, 225, 250, 300, 350, 400, 450, 500, 600, 700, 800, 1000, 1200, 1600, 2000, 2500, 3000, 4000, 5000, and 6000 amperes.

Additional standard ampere ratings for fuses shall be 1, 3, 6, 10, and 601.

Adjustable-Trip Circuit Breakers**
Breakers have external means for adjusting the current setting (long-time pickup setting), not meeting the requirements of Restricted Access breakers in below.

= the maximum setting possible.
Restricted Access Adjustable-Trip Circuit Breakers **(see fig.1)
Breakers that have restricted access to the adjusting means. Restricted access shall be defined as located behind one of the following:
  • Removable and sealable covers over the adjusting means,
  • Bolted equipment enclosure doors,
  • Locked doors accessible only to qualified personnel.


= the adjusted current setting (long-time pickup setting).
Notes:

* The use of fuses and inverse time circuit breakers with nonstandard ampere ratings shall be permitted.
** A combination of a current transformer and overcurrent relay shall be considered equivalent to an overcurrent trip unit.
- The set long-time pickup rating is the opposite to the instantaneous trip rating.




Fig (1)


Rule#1: overcurrent protection devices (OCPDs) Load

According to the National Electric Code, all overcurrent devices may be loaded to a maximum of 80% of their continuous ampere rating, unless they are specifically listed for 100%.





2- Selection of Circuit Breakers:





Multipole or Single-Pole Circuit breakers

Circuit breakers (as Overcurrent Device) shall open all ungrounded conductors of the circuit both manually and automatically. (see fig.2)
Exception: single-pole circuit breakers, with identified handle ties, shall be permitted as the protection for each ungrounded conductor as in the blow table.


Case
Condition
Multiwire Branch Circuits (see fig.3)
Serve only single-phase line-to neutral loads.

Grounded Single-Phase Alternating-Current Circuits (see fig.4)

  • individual single-pole circuit breakers rated 120/240 volts ac,
  • For line-to-line connected loads for single-phase circuits.


3-Phase and 2-Phase Systems (see fig.4)

  • For line-to-line loads in 4-wire, 3-phase systems or 5-wire, 2-phase systems,
  • The systems have a grounded neutral point and the voltage to ground does not exceed 120 volts.


3-Wire Direct-Current Circuits
  • Individual single-pole circuit breakers rated 125/250 volts dc,
  • For line-to-line connected loads for 3-wire, direct current circuits supplied from a system with a grounded neutral where the voltage to ground does not exceed 125 volts.








Fig (2)


Fig (3)

Fig (4)



Circuit breaker used as a switch

  • 15- or 20-ampere Circuit breakers used as switches in 120-volt and 277-volt fluorescent lighting circuits shall be listed and shall be marked SWD.
  • 15- or 20-ampere Circuit breakers used as switches in high-intensity discharge lighting circuits shall be listed and shall be marked as HID.
  • Circuit breakers marked “HID” can be used for switching both high-intensity discharge and fluorescent lighting loads; however, a circuit breaker marked “SWD” can be used only as a switching device for fluorescent lighting loads.







Circuit breaker Proper Voltage Rating

Circuit breakers must be properly applied within their voltage rating and as per the following:

Breaker Type
Permitted Application
Conditions
circuit breaker with a straight voltage rating (such as 240V or 480V)
Any circuit

the nominal voltage between any two conductors does not exceed the circuit breaker’s voltage rating.

A two-pole circuit
Breaker
In 3-phase, corner-
grounded delta circuit
Breakers Must be marked 1ph – 3ph
circuit breaker with a slash rating  (such as 120/240V or 480Y/277V)
in a solidly
grounded circuit

  • the nominal voltage of any conductor to ground does not exceed the lower of the two values of the circuit breaker’s voltage rating,
  • the nominal voltage between any two conductors does not exceed the higher value of the circuit breaker’s voltage rating.








Circuit Breaker without an Instantaneous Trip

Where a circuit breaker is utilized without an instantaneous trip, one of the following or approved equivalent means shall be provided:
  • Zone-selective interlocking,
  • Differential relaying,
  • Energy-reducing maintenance switching with local status indicator.


Zone-selective interlocking and Differential relaying do not require any manual intervention.





3- Selection of Fuses:



Disconnecting Means for Fuses

  • A single disconnect switch is permitted by 240.40 to serve more than one set of fuses, such as in multimotor installations or for electric space-heating equipment where the heating element load is required to be subdivided, each element with its own set of fuses.
  • The installation of cable limiters or similar current-limiting devices on the supply side of the service disconnecting means is permitted by 230.82(1). No disconnecting means is required on the supply side of such devices.






Permissible Usages for Fuses

fuses shall be permitted to be used in certain circuits as per the following Table:


Fuse Type
Ratings
Permissible Usages
Plug fuses of the Edison-base type (see fig.5)
Shall be classified at not over 125 volts and 30 amperes and below.

Plug fuses shall be permitted to be used in the following circuits:
  • Circuits not exceeding 125 volts between conductors (such as circuits supplied by 120/240-volt, single-phase, 3-wire systems and by 208Y/120-volt, 3-phase, 4-wire systems),
  • Circuits supplied by a system having a grounded neutral point where the line-to-neutral voltage does not exceed 150 volts.


Type S fuses (see fig.5)
Shall be classified at not over 125 volts and 0 to 15 amperes, 16 to 20 amperes, and 21 to 30 amperes.

Cartridge fuses and fuseholders of the 300-volt type (see fig.6)
Shall be classified at not over 300 volts.
shall be permitted to be used in the following circuits:
  • Circuits not exceeding 300 volts between conductors,
  • Single-phase line-to-neutral circuits supplied from a 3-phase, 4-wire, solidly grounded neutral source where the line-to-neutral voltage does not exceed 300 volts







Fig (5)



Fig (6)




4- Using Thermal Overload Relays:




Thermal Overload Relays

Thermal overload relays and other devices not designed to open short circuits or ground faults shall not be used for the protection of conductors against overcurrent due to short circuits or ground faults.
But the use of such devices shall be permitted to protect motor branch-circuit conductors from overload and shall be protected for short circuits or ground faults by fuses or circuit breakers or by a motor short-circuit protector.






4- Series Rated System

  • Where a circuit breaker is used on a circuit having an available fault current higher than its marked interrupting rating, A series rated system can be used.
  • A series rated system is a combination of circuit breakers or a combination of fuses and circuit breakers that can be applied at available short-circuit levels above the interrupting rating of the load side circuit breakers but not above that of the main or line side device.
  • Series rated systems can consist of fuses that protect circuit breakers or of circuit breakers that protect circuit breakers.
  • When a series rating is used, the switchboards, panelboards, and load centers must be marked for use with the series rated combinations.
  • Some conditions for using series rated system with certain application are listed in the following table:


Application
Conditions
Existing installation (where an increase in the available fault current (due to factors such as increases in transformer size, lowering of transformer impedances, and changes in utility distribution systems)

series rated system must be designed by a licensed professional engineer to ensure that used circuit breakers will:
  • remain closed during the interruption period of the fully rated OCPD installed on their line side,
  •  have an interrupting rating that is not less than the let-through current of an upstream protective device (such as a current-limiting fuse)


Motor Contribution (see fig.7)
In this Application, series rated system shall not be used where:
  • Motors are connected between the load side of the higher rated overcurrent device and on the line side of the lower-rated overcurrent device, 
  • The sum of the motor full-load currents exceeds 1 percent of the interrupting rating of the lower-rated circuit breaker.






Fig (7)



Parallel Use Of Fuses And Circuit Breakers

Fuses and circuit breakers shall be permitted to be connected in parallel where they are factory assembled in parallel and listed as a unit. Individual fuses, circuit breakers, or combinations thereof shall not otherwise be connected in parallel.





In the next Article, I will answer the following questions:

  • Where to locate the overcurrent protection devices (OCPDs)? 

Please, keep following.



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