Title: Reading and Audio-Visual Assignments
Course: WGF 107 DEVELOPMENT > Section: Content

Title | Other Electrical Safety Devices
Image | A miniature time-delay fuse used to protect electronic equipment, rated 3/10 amperes at 250 volts. 1 1/4 inches (about 32 mm) long

Sub-Title | Residual-current Devices (GFIs)
Content
A residual-current device (RCD), or residual-current circuit breaker (RCCB) or residual twin-direct current couplet (R2D2), is an electrical wiring device that disconnects a circuit whenever it detects that the electric current is not balanced between the energized conductor and the return neutral conductor. Such an imbalance may indicate current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit. A lethal shock can result from these conditions. RCCBs are designed to disconnect quickly enough to prevent injury caused by such shocks. They are not intended to provide protection against overcurrent (overload) or all short-circuit conditions.

In the United States and Canada, a residual-current device is most commonly known as a Ground Fault Circuit Interrupter (GFCI), Ground Fault Interrupter (GFI) or an Appliance Leakage Current Interrupter (ALCI). Ground Fault Condition is defined as: An unintentional, electrically conducting connection between an ungrounded conductor of an electrical circuit and the normally non-current-carrying conductors, metallic enclosures, metallic raceways, metallic equipment or earth.

Purpose and Operation

RCDs are designed to disconnect the circuit if there is a leakage current. By detecting small leakage currents (typically 5–30 milliamperes) and disconnecting quickly enough (<300 ms), they may prevent electrocution. They are an essential part of the automatic disconnection of supply (ADS), i.e. to switch off when a fault develops, rather than rely on human intervention, this is one of th essential tenets of modern electrical practice. There are also RCDs with intentionally slower responses and lower sensitivities, designed to protect equipment or avoid starting electrical fires, but not disconnect unnecessarily for equipment which has greater leakage currents in normal operation. To prevent electrocution, RCDs should operate within 25-40 milliseconds with any leakage currents (through a person) of greater than 30 milliamperes, before electric shock can drive the heart into ventricular fibrillation, the most common cause of death through electric shock. By contrast, conventional circuit breakers or fuses only break the circuit when the total current is excessive (which may be thousands of times the leakage current an RCD responds to). A small leakage current, such as through a person, can be a very serious fault, but would probably not increase the total current enough for a fuse or circuit breaker to break the circuit, and certainly not to do so fast enough to save a life.

RCDs operate by measuring the current balance between two conductors using a differential current transformer. This measures the difference between current flowing through the live conductor and that returning through the neutral conductor. If these do not sum to zero, there is a leakage of current to somewhere else (to earth/ground, or to another circuit), and the device will open its contacts. It is important to realise that operation does not require a fault current to return via the earth wire in the installation, the trip will operate just as well if the return path is via plumbing, contact with terra firma or any other current path. Automatic disconnection and a measure of shock protection is therefore still provided even if the earth wiring of the installation is damaged or incomplete.

Residual current detection is complementary to over-current detection. Residual current detection cannot provide protection for overload or short-circuit currents, except for the special case of a short circuit from live to ground (not live to neutral).
Video | Fuse Testing/Continuity Check http://www.youtube.com/watch?v=3nicyh6EUHc
Content
Testing Continuity and Insuring Safety

A GCM or ground continuity monitor (also called a ground integrity monitor or ground continuity tester) is an electrical safety device that monitors the impedance to ground of a temporary electrical circuit and can provide indication (or protective trip) in the event impedance rises to an unsafe value. A GCM is either an external testing device or a cord mounted device that measures the electrical continuity of a circuit’s path to ground.

A continuity tester is an item of electrical test equipment used to determine if an electrical path can be established between two points;^[1] that is if an electrical circuit can be made. The circuit under test is completely de-energized prior to connecting the apparatus.^[1]

The tester consists of an indicator in series with a source of electrical power - normally a battery, terminating in two test leads.^[1]^[2] If a complete circuit is established between the test-leads, the indicator is activated.^[2]

The indicator may be an electric light or a buzzer.^[1] This led to the term "buzzing out a circuit" (which means to test for continuity)^[^citation needed^]. Audible continuity buzzers or beepers are built into some models of multimeter.

A popular design has the tester combined with a standard flashlight. A phone connector or jack plug in the rear of the unit permits a set of test leads to be plugged in effecting a quick conversion between the two applications.

For situations where continuity testing must be done on high resistance circuits, or where delicate conductors and sensitive components that might be damaged by excessive current are present, a low voltage, low current device must be used.^[2]^[3] These typically use an op-amp and watch batteries to drive an LED as an indicator. These testers can be exquisitely sensitive; for example they will indicate if the test points are taken by both hands.

There are times when a simple continuity test fails to reveal the problem. For example, vibration-induced problems in automobile wiring can be extremely difficult to detect because a short or open is not maintained long enough for a standard tester to respond.

In these applications a latching continuity tester is used. A more complex device, it detects intermittent opens and shorts as well as steady-state conditions.^[3] These devices contain a fast acting electronic switch (generally a Schmitt trigger) forming a gated astable oscillator which detects and locks (latches) the indicator on an intermittent condition with a duration of less than a millisecond.
Links/Uploads
Residual-current Devices – Wikipedia http://en.wikipedia.org/wiki/Ground_Fault_Interruptor

See also[edit]

 | Energy portal

- Arc-fault circuit interrupter (AFCI)
- Earth leakage circuit breaker
- Insulation monitoring device
- Domestic AC power plugs and sockets

_______________________________________________

Antifuse Devices – Wikipedia  http://en.wikipedia.org/wiki/Antifuse

Ground Continuity – Wikipedia http://en.wikipedia.org/wiki/Ground_continuity_monitor

See also[edit]

- Cable tester
- Continuity test
- LED circuit
- Solenoid voltmeter
- Test light
Attributions | Images and Content courtesy of Wikipedia – The Free Internet Encyclopedia
Title | Activity Assignment – Discussion Board Introduction Question
Image |

Sub-Title | “State a situation where you engaged some form of electrical fuse replacement or testing equipment (who, what, when, where, and why). What did you want to identify? And what did you learn from that experience?”
Content
“State a situation where you engaged some form of electrical fuse replacement or testing equipment (who, what, when, where, and why). What did you want to identify? And what did you learn from that experience?”

This discussion question is meant to stimulate your idea of how a sustainable energy should be “conserved”, and how it should sustain your facility. If you have previously completed GFM101 – Introduction to Green Facilities, you should start to see the significance of energy conservation in green facilities management and areas of study in sustainable buildings’ and grounds’ energy.

Thus, please answer this question with regards to Fuse Replacement (every facility has some form of fuse/fuse-replacement recording… Some more oriented to recording than acquisition).

Throughout this discussion unit, please log into the discussion board, review the answers of your classmates and respond to at least one of the discussion answers with further comments or questions

If you do not respond to one of your peer’s questions, you will lose 5 points
Links/Uploads | (this field intentionally left blank)
Attribution | Image courtesy of Sustainable Facilities Tool – U.S. General Services Administration Content by David Bowman