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Electrical Wiring FAQ (Part 1 of 2)
Section - What does a fuse or breaker do? What are the differences?

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Top Document: Electrical Wiring FAQ (Part 1 of 2)
Previous Document: "grounding" versus "grounded" versus "neutral".
Next Document: Breakers? Can't I use fuses?
See reader questions & answers on this topic! - Help others by sharing your knowledge

	Fuses and circuit breakers are designed to interrupt the power
	to a circuit when the current flow exceeds safe levels.  For
	example, if your toaster shorts out, a fuse or breaker should
	"trip", protecting the wiring in the walls from melting.  As
	such, fuses and breakers are primarily intended to protect the
	wiring -- UL or CSA approval supposedly indicates that the
	equipment itself won't cause a fire.

	Fuses contain a narrow strip of metal which is designed to melt
	(safely) when the current exceeds the rated value, thereby
	interrupting the power to the circuit.  Fuses trip relatively
	fast.  Which can sometimes be a problem with motors which have
	large startup current surges.  For motor circuits, you can use
	a "time-delay" fuse (one brand is "fusetron") which will avoid
	tripping on momentary overloads.  A fusetron looks like a
	spring-loaded fuse.  A fuse can only trip once, then it must be

	Breakers are fairly complicated mechanical devices.  They
	usually consist of one spring loaded contact which is latched
	into position against another contact.  When the current flow
	through the device exceeds the rated value, a bimetallic strip
	heats up and bends.  By bending it "trips" the latch, and the
	spring pulls the contacts apart.  Circuit breakers behave
	similarly to fusetrons - that is, they tend to take longer to
	trip at moderate overloads than ordinary fuses.  With high
	overloads, they trip quickly.  Breakers can be reset a finite
	number of times - each time they trip, or are thrown
	when the circuit is in use, some arcing takes place, which
	damages the contacts.  Thus, breakers should not be used in
	place of switches unless they are specially listed for the

	Neither fuses nor breakers "limit" the current per se.  A dead
	short on a circuit can cause hundreds or sometimes even
	thousands of amperes to flow for a short period of time, which
	can often cause severe damage.

User Contributions:

Report this comment as inappropriate
Dec 21, 2011 @ 12:00 am
In a fire protection circuit, circuts are shown witha no example 6,8,4etc. what it mean?these circuits are connected between smode detector,junction box etc
Report this comment as inappropriate
Dec 24, 2011 @ 12:12 pm
My daughter dropped a small necklace behind her dresser. The necklace crossed a plug terminal and shorted the receptacle.
I bought a new receptacle and installed the same. I still have no power I suspect there could be a bigger problem,this is aluminum wiring.
I've killed the breaker and call an electrician but am curious as to what happened.P.s. there is a dimmer switch on the same circuit.
Report this comment as inappropriate
Feb 24, 2012 @ 11:11 am
Regarding new construction wiring and running 12/2 and 14/3 wire in the same box.

I have multiple switches to lights. Ran 12/2 and 14/3 into switch box and inspector wrote correction needed.

What should I have done instead?

thank you
Report this comment as inappropriate
Nov 26, 2012 @ 9:21 pm
Does a grounding electrode facilitate the operation of a OCPD, to clear a ground fault ?
Report this comment as inappropriate
Mar 18, 2013 @ 10:10 am
Assuming you are installing two switches in a two switch box, you probably should have used 14/2 and 14/3 instead of replacing 14/2 with 12/2. If you are only installing one switch in a one switch box, you should only have one cable in the box.
P k
Report this comment as inappropriate
Jan 26, 2014 @ 10:10 am
I prefer to use nothing smaller than12 awg /the smallest sized wire on a circuit determines the allowable ampacity
Ex: 15 amp-14awg. 12awg-20amp only rule for thumb other factors such as continuous load,heating and others if you do not know the safe NEC rules then please call a qualified journeyman Electrician better be safe

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Top Document: Electrical Wiring FAQ (Part 1 of 2)
Previous Document: "grounding" versus "grounded" versus "neutral".
Next Document: Breakers? Can't I use fuses?

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Last Update March 27 2014 @ 02:11 PM