DAY 17 CHAPTER 28 OVERCURRENT PROTECTION FUSES AND CIRCUIT BREAKERS

FIVE CIRCUIT CONDITIONS

1. NORMAL LOADING2. OVERLOAD3. SHORT CIRCUIT4. GROUND FAULT5. OPEN

NORMAL CIRCUIT

THE CONDUCTORS CAN SAFELY CARRY THE CURRENT. THEY DO NOT GET HOT. THE 15A FUSES DO NOT OPEN.

CONDUCTORS RATED AT15ACONDUCTOR RESISTANCE=0.001Ω

15A FUSE

15A FUSE

LOAD RESISTANCE240V SOURCESOURCE R=1mΩ 24Ω

AV

mmm

V

R

EI CIRCUITTHRU 99875.9

003.24

240

11124

240

9.99875A

OVERLOADED CIRCUIT

THE CONDUCTORS BEGIN TO GET HOT, BUT THE 15A FUSES WILL OPEN BEFORE THE CONDUCTORS ARE DAMAGED.

CONDUCTORS RATED 15ACONDUCTOR RESISTANCE=0.001Ω

15A FUSE

15A FUSE

240V SOURCESOURCE R=1mΩ 24Ω

19.995A

24Ω

122

24

N

RR NLOAD

AV

mmm

V

R

EI CIRCUITTHRU 995.19

003.12

240

11112

240

SHORT CIRCUIT

THE CONDUCTORS GET EXTREMELY HOT. THE INSULATION AND THE CONDUCTORS WILL MELT UNLESS THE FUSE OPENS QUICKLY.

CONDUCTORS RATED 15ACONDUCTOR RESISTANCE=0.001Ω

15A FUSE

15A FUSE

SHORT CIRCUIT PATH

240V SOURCESOURCE R=1mΩ 24Ω

80,000A

AV

mmm

V

R

EI CIRCUITTHRU 000,80

003.0

240

111

240

LOAD RESISTANCE

024

0

024

024

21

21

RR

RRRLOAD

GROUND FAULT

SOURCE

HOT CONDUCTOR

GROUNDED CONDUCTOR

WHEN A HOT CONDUCTOR COMES IN CONTACT WITH METAL AND THE RETURN PATH HAS HIGH RESISTANCE THE BREAKER WILL NOT TRIP. THE METAL WILL HAVE THE SAME VOLTAGE AS THE HOT.THE VAULE OF RESISTANCE CAN VARY GREATLY DUE TO THE DIFFERENCE COMPOENTS ALONG THE CIRCUIT PATH, MAKING IT HARD TO FIND THE GROUND FAULT CURRENT VALUE.

OPEN CIRCUIT

THE CIRCUIT HAS AN OPEN, NO CURRENT WILL FLOW.

CONDUCTORS RATED 15ACONDUCTOR RESISTANCE=0.001Ω

15A FUSE

15A FUSE

LOAD RESISTANCE240V SOURCESOURCE R=1mΩ

0A

24Ω

AV

R

EI CIRCUITTHRU 0

240

OPEN

ANY OPEN HAS INFINITE RESISTANCE

FUSES/CIRCUIT BREAKERS: PROTECT CIRCUIT FROM CURRENT OVERLOAD.

SHORT : UNWANTED PATH THAT CURRENT TAKES.

PARTIAL SHORT :ONLY PART OF LOAD IS SHORTED OUT.

DEAD SHORT : ALL OF THE LOAD IS SHORTED OUT.

SCREW BASE FUSES HAVE RATINGS FROM 0 TO 30AMPS

Fuses have two types of bases, the Edison Base (Type T fuses) and the Rejection Base (Type S fuses), also known as a "tamper-proof base". The Edison Base Type T fuse works with the typical socket used in the fuse box seen in older homes and the fuse base looks like a light bulb base. A Rejection Base Type S fuse actually consists of two components, an adapter base that screws and locks into the Edison socket in the fuse box, and the fuse itself. Each different amperage rating of a Type S fuse has a matching socket base adapter with a different sized thread that prevents mismatching (rejection base). It stops a person from putting a 20 Amp rated fuse in a 15 Amp circuit.

REJECTION BASE, TYPE S FUSE

The Bussmann S Series Plug Fuse is for all-purpose applications. It's like two fuses in one. Features include a simple link element for short-circuits and dangerous overloads, plus a series-connected element which lets the harmless overload starting currents of motors pass without opening. It uses less energy, operates cooler, and provides superior protection.

DUAL ELEMENT FUSE

CARTRIDGE FUSES ARE RATED FROM 1/8 T0 600 AMPS IN BOTH 250V AND 600V

CARTRIGE FUSES CAN BE USED AS THE MAIN DICONNECT MEANS

115KV FUSE

FUSES

CIRCUIT BREAKERS: TWO TYPES: THERMAL,MAGNETIC

THERMAL:CAN BE RESET MANUALLY OR AUTOMATIC. AUTOMATIC RESET OCCURS AFTER BREAKER HAS COOLED.

Circuit Breaker: A device designed to open and close a circuit by nonautomatic means and to open the circuit automatically on a predetermined overcurrent without damage to itself when properly applied within its rating.

SINGLE POLEDUAL POLE

TWO POLETHREE POLE BREAKER WITH HANDLE TIE

MAGNETIC CB: WHEN A OVERLOAD OCCURS A BIMETALLIC ELEMENTS UNLATCHES THE INNER TRIP MECHANISM. A MOMENTARY OVERLOAD DOES NOT CAUSE THE BREAKER TO TRIP. IF THE OVERLOAD IS HEAVY, A MAGNETIC COIL CAUSES IT TO TRIP.

Circuit Breaker Animation & Working http://www.youtube.com/watch?v=GSh0f94JwaA

MULTI WIRE BRANCH CIRCUITS MUST HAVE HANDLE TIESON CIRCUIT BREAKER FOR SIMULTANEOUS TRIPPING.

240.24 Location in or on Premises.(A)Accessibility. Overcurrent devices shall be readily

accessible and shall be installed so that the center of the grip of the operating handle of the switch or circuit breaker.

(D) Not in Vicinity of Easily Ignitible Material. Overcurrentdevices shall not be located in the vicinity of easilyignitible material, such as in clothes closets.

(E) Not Located in Bathrooms. In dwelling units andguest rooms or guest suites of hotels and motels, overcurrentdevices, other than supplementary overcurrent protection,shall not be located in bathrooms.

(F) Not Located over Steps. Overcurrent devices shall notbe located over steps of a stairway.

230.79(C) One-Family Dwellings. For a one-family dwelling,the service disconnecting means shall have a rating of notless than 100 amperes, 3-wire.

OCPD (OVERCURRENT PROTECTION DEVICES) LOCATIONS

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