Overload currents

Pulses of 50-125-Hz have been used as an analgesic in dentistry. The mechanistic understanding of these effects (which are related to those of acupuncture) has not been pursued. Perhaps the wave forms are effective in stimulating morphinelike compounds. Alternatively, the applied currents may simply overload currents carrying the natural pain signals to the brain. 

For example, if we short the outputs of a dc-dc converter, we know it is unable to regulate, however hard it tries. In this fault condition ( open-loop ), the momentary overload current within the circuit can be tamed (or mitigated) a great deal by the very presence of certain identifiably friendly parasitics. 

IEC60364 Part 4, section 433, applies to cables and consumer equipment that are protected against overloading current by a fuse or relay device at the source of supply. This requirement should not be confused with protection against short-circuit currents that are disconnected in a short period of time. Overloading currents tend to cause the protective device to disconnect the circuit only after a long period of time has passed e.g. tens of minutes, one hour. The standard defines three particular currents, / , Ig and h as follows. 

Voltage (shall be 415 3 %) and current available at terminals (shall be suitable for the maximum current rating and some overload current). 

To provide overcurrent protection it is necessary to provide a device which will disconnect the supply automatically before the overload current can cause a rise in temperature which would damage the installation. A fuse or MCB would meet this requirement. 

An overcurrent may be an overload current, or a short-circuit current. An overload current can be defined as a current which exceeds the rated value in an otherwise healthy circuit. Overload currents usually occur because the circuit is abused or because it has been badly designed or modified. A short-circuit is an overcurrent resulting from a fault of negligible impedance connected between conductors. Short-circuits usually occur as a result of an accident which could not have been predicted before the event. 

Ordinary person Overload current Passive infra-red (PIR) detectors... 

An overload current can be defined as a current which exceeds the rated value in an otherwise healthy circuit. 

Fuse wire A fine-gauge wire made of an alloy that overheats and melts at the relatively low temperatures produced when the wire carries overload currents. When used in a fuse, the wire is called a fuse (or fusible) link. 

Motor circuit switch A switch, rated in horsepower, capable of interrupting maximum operating overload current of a motor of the same horsepower rating at rated voltage. 

Fuses. Several types of fuses are in use standard one-time, time-delay (or dual-element), and current-limiting. The standard one-time fuse has no time-delay feature. The time-delay dual-element fuses provide time delay where a heavy overload current might exist for a short time (normal motor starting).This type of fuse will not open the circuit during this brief time however, if the overload continues, the fuse wiU cause the circuit to open. In a short-circuit condition, this type of fuse wiU open the circuit in an extremely short time. 

Electrical protective devices are used to protect machines and equipment from damage due to values of voltage or current exceeding design levels. They are also used to protect individuals from the risk of injury, by isolating electrical faults in part of the system. Electrical hazards associated with workshop machinery and tools may be caused by overload current, short-circuit current, or electric shocks to individuals arising from direct or indirect contact with live conductors. Overload current is an excessive current... 

Protection Against Overload Current. In order to protect against overload or excess current, the overloaded circuit, or part of the circuit, must be disconnected quickly. The most simple and common form of overload protection is the fuse. The fuse will offer close protection as it should melt within 4 hours if current of 1 5 times its rating flows (e.g. fuses to BS 88). The fuse should be located in the live side of the circuit only, and not in the earthed neutral side, because if the neutral fuse melts first, the apparatus will cease to operate and will appear dead , while in fact still being dangerously live. Circuit-breakers are also used for overload protection as an alternative to fuses. A circuit-breaker is an automatic switching device, which operates instantaneously when a significant overload current (typically 1.5 times rated current) is detected. Circuit-breakers are easily reset after a trip, but are more expensive than fuses. 

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