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DAY 17 CHAPTER 28 OVERCURRENT
PROTECTION FUSES AND CIRCUIT BREAKERS
FIVE CIRCUIT CONDITIONS
1. NORMAL LOADING
2. OVERLOAD
3. SHORT CIRCUIT
4. GROUND FAULT
5. OPEN
NORMAL CIRCUIT
THE CONDUCTORS CAN SAFELY CARRY THE CURRENT.
THEY DO NOT GET HOT. THE 15A FUSES DO NOT OPEN.
CONDUCTORS RATED AT15A
CONDUCTOR RESISTANCE=0.001Ω
15A FUSE
240V SOURCE
SOURCE R=1mΩ
9.99875A
24Ω
LOAD RESISTANCE
15A FUSE
I THRU CIRCUIT 
E
240V
240V


 9.99875 A
R 24  1m  1m  1m 24.003
OVERLOADED CIRCUIT
THE CONDUCTORS BEGIN TO GET HOT, BUT THE 15A FUSES
WILL OPEN BEFORE THE CONDUCTORS ARE DAMAGED.
CONDUCTORS RATED 15A
CONDUCTOR RESISTANCE=0.001Ω
RLOAD 
15A FUSE
240V SOURCE
SOURCE R=1mΩ
19.995A
24Ω
RN 24

 12
N
2
24Ω
15A FUSE
I THRU CIRCUIT
E
240V
240V
 

 19.995 A
R 12  1m  1m  1m 12.003
SHORT CIRCUIT
THE CONDUCTORS GET EXTREMELY HOT. THE INSULATION AND THE
CONDUCTORS WILL MELT UNLESS THE FUSE OPENS QUICKLY.
LOAD RESISTANCE
CONDUCTORS RATED 15A
CONDUCTOR RESISTANCE=0.001Ω
15A FUSE
240V SOURCE
SOURCE R=1mΩ
RLOAD 
R1  R2 24  0
0


0
R1  R2
24  0
24
80,000A
24Ω
15A FUSE
SHORT CIRCUIT PATH
I THRU CIRCUIT
E
240V
240V
 

 80,000 A
R 1m  1m  1m 0.003
GROUND FAULT
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.
HOT CONDUCTOR
SOURCE
GROUNDED CONDUCTOR
OPEN CIRCUIT
THE CIRCUIT HAS AN OPEN, NO CURRENT WILL FLOW.
CONDUCTORS RATED 15A
CONDUCTOR RESISTANCE=0.001Ω
15A FUSE
OPEN
240V SOURCE
SOURCE R=1mΩ
24Ω
15A FUSE
ANY OPEN HAS INFINITE RESISTANCE
LOAD RESISTANCE
0A
I THRU CIRCUIT 
E 240V

 0A
R

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
DUAL ELEMENT 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.
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.
DUAL POLE
SINGLE POLE
THREE POLE BREAKER WITH HANDLE TIE
TWO POLE
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 TIES
ON CIRCUIT BREAKER FOR SIMULTANEOUS TRIPPING.
OCPD (OVERCURRENT PROTECTION DEVICES) LOCATIONS
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. Overcurrent
devices shall not be located in the vicinity of easily
ignitible material, such as in clothes closets.
(E) Not Located in Bathrooms. In dwelling units and
guest rooms or guest suites of hotels and motels, overcurrent
devices, other than supplementary overcurrent protection,
shall not be located in bathrooms.
(F) Not Located over Steps. Overcurrent devices shall not
be 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 not
less than 100 amperes, 3-wire.
Noalox Anti-Oxidant Compound
Reduces galling and seizing on aluminum conduit joints
Suspended zinc particles penetrate and cut aluminum oxide
Carrier material excludes air to prevent further oxidation
Improves service life of aluminum electrical applications