What do fuzes NOT protect against? lighting surges, short circuits, over loading or large ground fault?
OK…I will try to keep it brief…I am sure it can be elaborated on in more detail…P.S…I think you mean FUSES
A fuse in its nature is simply device that has a metal bridge per se in the vacuum that when ampacity exceeds the rating of the metal it melts for a lack of better term opening the circuit…a Circuit breaker is simular but the metal actually bends…and allows the spring to pull the circuit closed…
The funny thing is and what eaton and Sq.D dont tell you that is usually their warranty only lasts for the first TRIP…lol…anyway I digress…
These are classified in the NEC as OCPD ( Over Current Protection Devices ) and they are designed as such…not ground fault protection, not surge protection and so on…
Their primary purpose is to protect the wiring and equipment they are installed within…not the person on the other end…because damaging amperes in a SHORT can exceed hundereds and thousands of amps in a very short time…thus the rating in amps…
While Ground Fault devices have their own purpose and will react MUCH…MUCH faster than a OCPD will…In cases where their is a fault onto the wiring it will increase the amps and eventually blow the fuse or trip the breaker…but in a ground fault situation breakers are simply NOT fast enough…again it takes LESS than 1/10th of an AMP to kill you…in fact less than 6MA can cause defib.
The primary protection of the fuse and circuit breaker is to reduce the chance of heat buildup and damage to the wiring and equipment protected by the fuse…
Now here is a little bit on Circuit Breakers…
present-day residential circuit breakers were designed to protect wiring from excessive heating by opening automatically when an overload condition was present. These breakers typically contain a bimetal and magnetic trip element.
During low current over-loads, heating of the bimetal element causes the breaker to unlatch the contact-separation mechanism, turning power off in the affected circuit. For high overcurrent conditions, such as short circuits, the high magnetic field associated with high current flow causes the “instantaneous tripping” of the breaker-mechanism.
The breaker response time is a function of the circuit current and time, with faster response at higher currents. In particular, the circuit breaker Time versus Current characteristics, is selected to prevent conductor damage. Present overcurrent devices, such as circuit breakers and fuses, represent a major safety feature in today’s residences.
They prevent excessive temperatures in the conductors or conductor insulation and they reduce the incidence of household fires, to a great extent, through their prevention of overheating effects. The response time of present day circuit breakers is determined solely by the duration and magnitude of the circuit overcurrent.
This response is adequate to protect the wiring if the circuit wiring integrity has been maintained. However, once this integrity has been compromised via broken wires or deteriorated insulation, electric arcs can occur. These arcs are characterized by a plasma flame which can release temperatures in excess of 6000ºC.
Fuses and breakers do nothing to protect people from shocks or electronics from surges.
Yep…I think I had a neat article explaining them…Yeppers…basically as Greg said…and is often misunderstood…OCPD are for that purpose only…let me see if I can upload the PDF…
lol…kinda my round about way of saying that Greg…lol
Great Information Paul!
So in an application like a HVAC OCPD the “trigger” time with respect to initial current draw at the moment of activation… Compressor kick’s in, the OCPD is a delayed trigger when the operating current “returns” to “normal” current draw?
Yes, actually even a OCPD that is a CB has a bit of delay in it due to the effect of the metal strip. Obviously also the reason they in many cases allow a larger breaker on the circuit than the wire itself…to compensate for this momentary delay associated with the compressor…and is usually already factored in with the nameplate rating.