Overcurrent devices

Overcurrent devices are devices that automatically open electric circuits in predetermined overload and short circuit states, either by metal-fusing or electromechanically (e.g. with switches).

predetermined: done in advance
overload: if a device exceeds its limit of current
- predetermined overload: a current level installed in advance, if exceeded, shuts down the motor with a relay or fuse, to stop overheating/damage
to fuse: to melt
electromechanical: an electrically operated device

Unlike an overload, overcurrent refers to any case if current limits is exceeded.

Thought interchangeable, overcurrent is a broader term and overload is specifically for "exceeded load".

Examples of overcurrent devices

Overcurrent devices include fuses, circuit breakers, relay, and solid state power switches.

an solid state power switches

[5] Fuses

As its name says, fuse will "metal-fuse" (melt) if current exceeds the limit to break the circuit to stop electricity flow.

They're small cylindrical object with 2 metal end caps, connected by a thin metal wire inside, often enclosed in glass.

Circuit breakers

[3] Circuit breaker are electrical switches that automatically stops electrical circuits to stop damage in short circuits or overloads.

They're used in homes, offices, and industries.

Overcurrent (O/C) ratings

Overcurrent (O/C) ratings are ratings for different wire sizes made of copper or aluminum.

Numbers indicate the max current (in amps) a wire can safely have in some cases.

E.g.

O/C for Single Cu R75 8 AWG = 70 A
O/C for Single Al R90 8 AWG = 60 A
O/C for Single Cu R90 10 AWG = 30 A
O/C for Single Al R75 12 AWG = 15 A

Breakdowns:

Single Cu R75 8 AWG = 70 A: 1 copper wire (8 AWG) of a 75°C rating can have up to 70 amps
Single Al R90 8 AWG = 60 A: 1 aluminum wire (8 AWG) of a 90°C rating can have up to 60 amps
Single Cu R90 10 AWG = 30 A: 1 copper wire (10 AWG) of a 90°C rating can have up to 30 amps
Single Al R75 12 AWG = 15 A: 1 aluminum wire (12 AWG) of a 75°C rating can have up to 15 amps

Overload

An overload is excess current, more current than a circuit/equipment can safely handle.

An electric overload occurs when too much current passes through electric wires. Wires heat and can melt, risking fire.

This is a classic overloaded circuit and...

...this can occur.

Characteristics

Usual overloads are >100% and <500% of circuit rating.

Overload causes:

excess heat

insulation failure

Insulation failures cause:

equipment failure
fires

Other overloads Loads exceeding a motor’s rated horsepower causes to draw more than rated current, causing overheat.

horsepower: powers engines make

A good winding motor sheen and colour.

Winding damaged by overload - Note colour. We can tell by colors that the red one will fail.

motor winding: an insulated electrical conductors coil wrapped around an electric motor's magnetic core. If energized by current, it creates magnetic field to rotate the motor's rotor.

"Octopus"

In electrical overcurrent protection, "octopus" often to refer a circuit breaker type or protection scheme with protection elements. It might refer to a system where several protection devices (like fuses or breakers) are grouped together to safeguard various parts of an electrical network.

Example

In this case, an “octopus” is used to connect 3 appliances to a single 120-volt, 15 amp outlet.

total load = 50 + 1000 + 800 or 1850 W

At 120 volts this needs a current of 15.4 amps, exceeding both the wire and overcurrent device's ratings.

Power bars and “octopi” can create a hazard if the devices's ratings are exceeded.

Short circuits

Short circuits is when current finds an unintended path to ground or between wires, the worst case for a circuit. Current may exceed 20 times the circuit rating.

Short circuit example 1

A circuit with 2 x 25 m of 14 AWG copper conductor.

With a 100 W lamp the current is 0.83 amps.

Example 2

The same circuit with a short at the lamp.

Now the current is only limited by the conductors' resistance, ~0.4143 Ω.

How much current flows now?

Example 3

I = E/R

= 120 / 0.4143

= 290 A

This is 12-9 times the conductor's rating.

How much heat is made?

P = E x I = 120 x 290 = 34.8 kW

Example 4

To put 34.8 kW in context, I heated my house with a 20 kW

electric furnace.

Such fault must be cleared fast.

What would occur if the distance to the fault is shorter?

Swimming up short circuits

Often electrical design focuses on short circuits.

If the systems can withstand a short, it handles all other situations.

Larger an electrical supply, larger the short circuit currents.

a short-circuited AA battery maekes little heat
a transformer's short circuit that supplies a house cause an explosion

Section 14 - Protection and control

General requirements

14-010 Protective and control devices required

Electrical apparatus and ungrounded conductors shall, except as otherwise provied for in this Section or in other Sections dealing with specific equipment, be provided with

(a) devices for the purpose of automatically opening the electrical circuit thereto, (i) if the current reaches a value that will produce a dangerous temperature in the apparatus or conductor; and

Inverse time characteristics

All overcurrent devices have “inverse-time” traits.

Small over currents not lasting long don’t open an overcurrent device--critical to start motors.

Inverse time characteristics 1

The curve shows the current needed to open a 100 amp fuse.

The curve never reach the 100 amp line (green).

Scales are logarithmic.

Inverse time characteristics 2

orange line = ~300 amps.

Horizontally from where the 300 amp line intersects the curve we get 10 seconds.

300 amps will take about 10 seconds to open a 100 amp fuse.

Inverse time characteristics 3

Now the orange line is about 1000 amps.

Horizontally from where the 1000 amp line intersects the curve, it's ~400 milliseconds.

Fault current increase, time to open decrease.

14-012 Ratings of protective

In circuits of 750 V and less,

Electrical equipment required to interrupt fault currents shall have ratings sufficient for the voltage employed and for the fault current that is available at the terminals; and

Electrical equipment required to interrupt current at other than fault levels shall have ratings sufficient for

the voltage employed and for the current it must interrupt.

Disconnecting means

14-402 Disconnecting means needed for fused circuits (see Appendix B)

Circuits protected by fuses shall be equipped

with disconnecting means, integral with or

adjacent to the fuseholders, whereby all live

parts for mounting fuses can be readily and

safely made dead;

Fused disconnect

References

Quizzes

[Q1] Flashcard
[E1] Exercises

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