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Tuesday, June 28, 2011
Friday, February 11, 2011
Safety Alert
Mine Safety Report No: SA05-12
File No:05/4589
Comet ID:317523671001
Prepared by: Bob Kennedy IEE Wollongong
Phone: 0438467992
Date Created: 3 September 2005
SAFETY ALERT
Electric shock from a light fitting.
INCIDENT
A mineworker received an electric shock when his forearm made contact with a
flameproof Burn Brite fluorescent light fitting (Certificate No AUS EX 424X) on a
continuous miner in the hazardous zone of a coal mine. The victim was
transported to hospital in accordance with the mine’s electric shock protocol and
was allowed to leave after medical tests and examination determined that no
permanent injury had occurred as a result of the shock and there was minimal
risk of delayed arrhythmia.
The presence of electricity external to the flameproof enclosure also presented a
potential ignition source of methane
CIRCUMSTANCES
The source of voltage was from a 110 volt power supply located in the master
control enclosure on the continuous miner. The Burn Brite light fitting case was
plastic; however three brass threaded inserts are embedded in the boss
assembly to enable cable glands to be fitted. These metal inserts are not
connected to earth, and any metal gland screwed in may not be earthed (see
photograph below)
The light fitting was mounted vertically which allowed water to enter and
accumulate as a result of a missing “O” ring between the boss and nut assembly
of the light fitting. The missing “O” ring constitutes a failure of the IngressMine Safety Report No: SA05-12
File No:05/4589
Comet ID:317523671001
Prepared by: Bob Kennedy IEE Wollongong
Phone: 0438467992
Date Created: 3 September 2005
he presence of water in the light fitting caused a fault current to flow to the
VESTIGATION
ingress protection,
rn Brite light fitting, and
com e of the coal
hazards were;
was able to deliver an electric shock to a mineworker,
2. xplosion-protected equipment failed to contain the electrical energy
ECOMMENDATIONS
ay also apply to other light fittings and on other
Protection (IP). (Note: The “O” ring may become dislodged during re-assembly of
the light fitting after maintenance). The outer case collar (flamepath) was
cracked (see photograph below) and the poly-carbonate case was also cracked.
T
earth screen of its supply cable. This imposed a touch voltage onto the
unearthed external cable gland. The fault current was below the magnitude
necessary to cause the electrical protection to operate. The circuit protection is a
10-ampere fuse and a toroidal earth-leakage relay set to trip at 500 milliamps,
within 250 milliseconds. The light fitting continued to operate with the water
present.
IN
The three factors of:
1. A failure of the
2. Unearthed external metal on the Bu
3. The 500 mA/250 ms setting of the earth leakage protection,
bined to present two separate hazards in the hazardous zon
mine.
These
1. A touch voltage
AND
The e
within the enclosure under fault conditions creating a potential source of
ignition of methane.
R
The issues raised herein m
machinery at NSW mines. Mine Safety Report No: SA05-12
File No:05/4589
Comet ID:317523671001
Prepared by: Bob Kennedy IEE Wollongong
Phone: 0438467992
Date Created: 3 September 2005
Manufacturers of light fittings and continuous miners, should review the risks to
safety from their products and to promptly stipulate earthing, protection,
maintenance and inspection requirements to their customers as required by the
NSW OHS Act.
All mines should assess the risks to safety of the Burn Brite light fitting in a
manner consistent with clause 6 of the Coal Mines (General) Regulation to
decide if continued use is appropriate, and if so, under what conditions of use,
including inspection, environment, earth leakage protection settings and
inspection and overhaul schedules.
All mines should review the training and competence requirements of persons
responsible for maintenance of the light fitting to:
• Prevent mistaken assembly,
• Identify cracks in the flameproof assembly.
• Ensure adequate earthing
All mines should review the inspection, testing and maintenance of all electrical
equipment to include the checking of the integrity of earthing of exposed metal
parts and earth continuity testing.
Rob Regan
DIRECTOR
MINE SAFETY OPERATIONS BRANCH
NSW DEPARTMENT OF PRIMARY INDUSTRIES
View more safety alerts at www.minerals.nsw.gov.au. If you would like to receive safety alerts by email, send your details
to safetyalert@minerals.nsw.gov.au .
Source
File No:05/4589
Comet ID:317523671001
Prepared by: Bob Kennedy IEE Wollongong
Phone: 0438467992
Date Created: 3 September 2005
SAFETY ALERT
Electric shock from a light fitting.
INCIDENT
A mineworker received an electric shock when his forearm made contact with a
flameproof Burn Brite fluorescent light fitting (Certificate No AUS EX 424X) on a
continuous miner in the hazardous zone of a coal mine. The victim was
transported to hospital in accordance with the mine’s electric shock protocol and
was allowed to leave after medical tests and examination determined that no
permanent injury had occurred as a result of the shock and there was minimal
risk of delayed arrhythmia.
The presence of electricity external to the flameproof enclosure also presented a
potential ignition source of methane
CIRCUMSTANCES
The source of voltage was from a 110 volt power supply located in the master
control enclosure on the continuous miner. The Burn Brite light fitting case was
plastic; however three brass threaded inserts are embedded in the boss
assembly to enable cable glands to be fitted. These metal inserts are not
connected to earth, and any metal gland screwed in may not be earthed (see
photograph below)
The light fitting was mounted vertically which allowed water to enter and
accumulate as a result of a missing “O” ring between the boss and nut assembly
of the light fitting. The missing “O” ring constitutes a failure of the IngressMine Safety Report No: SA05-12
File No:05/4589
Comet ID:317523671001
Prepared by: Bob Kennedy IEE Wollongong
Phone: 0438467992
Date Created: 3 September 2005
he presence of water in the light fitting caused a fault current to flow to the
VESTIGATION
ingress protection,
rn Brite light fitting, and
com e of the coal
hazards were;
was able to deliver an electric shock to a mineworker,
2. xplosion-protected equipment failed to contain the electrical energy
ECOMMENDATIONS
ay also apply to other light fittings and on other
Protection (IP). (Note: The “O” ring may become dislodged during re-assembly of
the light fitting after maintenance). The outer case collar (flamepath) was
cracked (see photograph below) and the poly-carbonate case was also cracked.
T
earth screen of its supply cable. This imposed a touch voltage onto the
unearthed external cable gland. The fault current was below the magnitude
necessary to cause the electrical protection to operate. The circuit protection is a
10-ampere fuse and a toroidal earth-leakage relay set to trip at 500 milliamps,
within 250 milliseconds. The light fitting continued to operate with the water
present.
IN
The three factors of:
1. A failure of the
2. Unearthed external metal on the Bu
3. The 500 mA/250 ms setting of the earth leakage protection,
bined to present two separate hazards in the hazardous zon
mine.
These
1. A touch voltage
AND
The e
within the enclosure under fault conditions creating a potential source of
ignition of methane.
R
The issues raised herein m
machinery at NSW mines. Mine Safety Report No: SA05-12
File No:05/4589
Comet ID:317523671001
Prepared by: Bob Kennedy IEE Wollongong
Phone: 0438467992
Date Created: 3 September 2005
Manufacturers of light fittings and continuous miners, should review the risks to
safety from their products and to promptly stipulate earthing, protection,
maintenance and inspection requirements to their customers as required by the
NSW OHS Act.
All mines should assess the risks to safety of the Burn Brite light fitting in a
manner consistent with clause 6 of the Coal Mines (General) Regulation to
decide if continued use is appropriate, and if so, under what conditions of use,
including inspection, environment, earth leakage protection settings and
inspection and overhaul schedules.
All mines should review the training and competence requirements of persons
responsible for maintenance of the light fitting to:
• Prevent mistaken assembly,
• Identify cracks in the flameproof assembly.
• Ensure adequate earthing
All mines should review the inspection, testing and maintenance of all electrical
equipment to include the checking of the integrity of earthing of exposed metal
parts and earth continuity testing.
Rob Regan
DIRECTOR
MINE SAFETY OPERATIONS BRANCH
NSW DEPARTMENT OF PRIMARY INDUSTRIES
View more safety alerts at www.minerals.nsw.gov.au. If you would like to receive safety alerts by email, send your details
to safetyalert@minerals.nsw.gov.au .
Source
Wednesday, January 12, 2011
How to choose a Temperature Switch
A temperature switch is a switch that is responsive
to temperature changes. Temperature switchesgenerally are provided with a temperature responsive element which will open or close a switch when a predetermined minimum pressure or temperature is sensed by the responsive element. For protection against thermal overload,semiconductor switches are provided with integrated temperature sensors. The temperature sensors acquire the temperature of the power switch and convert this into a temperature-dependent, analog signal which then can be interpreted in a circuit. Temperature sensitive switches, such as a thermostat, typically comprise a temperature sensor which is used to open or close electrical contacts at specified temperatures. A bimetal strip of dissimilar metals is used as the sensing element for temperature sensitive switches. Temperature sensitive switches are often used for thermal protection purposes. If a device gets too hot, the temperature sensitive switch opens the electrical circuit, thereby eliminating power to the circuit. For example, temperature responsive tip-switches are particularly useful in connection with electric heaters.
Source
to temperature changes. Temperature switchesgenerally are provided with a temperature responsive element which will open or close a switch when a predetermined minimum pressure or temperature is sensed by the responsive element. For protection against thermal overload,semiconductor switches are provided with integrated temperature sensors. The temperature sensors acquire the temperature of the power switch and convert this into a temperature-dependent, analog signal which then can be interpreted in a circuit. Temperature sensitive switches, such as a thermostat, typically comprise a temperature sensor which is used to open or close electrical contacts at specified temperatures. A bimetal strip of dissimilar metals is used as the sensing element for temperature sensitive switches. Temperature sensitive switches are often used for thermal protection purposes. If a device gets too hot, the temperature sensitive switch opens the electrical circuit, thereby eliminating power to the circuit. For example, temperature responsive tip-switches are particularly useful in connection with electric heaters.
Tips for choosing the right temperature switch
A temperature switch responds immediately to temperature change. A temperature switch will usually have some sort of component that responds by opening and closing a switch once the established minimum pressure or temperature is recognized by the responding component.
A thermostat is one example of a temperature switch. It is usually includes a temperature sensor. The sensor opens and closes electrical contacts once a present temperature is reached.
Which temperature switch is right for your application?
Temperature sensitive switches are commonly used as a form of thermal protection. If an appliance becomes too hot, the temperature switch will open the electrical circuit. This will immediately shut off power to the circuit.
Trying to find the right temperature sensor to use in a system can be difficult. If you do not do your research, you could spend a lot of money searching for the right one. It is estimated that 55 percent of all electronic system failures are the result of a temperature related cause.
Programming the switch
A thermal device can be programmed to activate at several different temperature thresholds. The temperature switch monitors the temperature of an electrical appliance and maintains control over the amount of power that is used. If the temperature is programmed to low it will reduce the appliance's performance and efficiency. It will also create an environment for condensation to develop. This can lead to insulation breakdown and corrosion.
Maintaining the temperature threshold
The sensor sends out a signaling current that matches the absolute temperature. The rating, or value, of the electrical current determines what the temperature threshold of the temperature switch should be. The typical temperature switch design is complicated and can consume a lot of power. There is a new product called a sub threshold CMOS temperature switch available. Its function is simpler, and it uses less power. However, it is not as stable as the more common temperature switch found in most electrical devices when it comes to maintaining threshold temperature.
The functions of thermal protection
The temperature switch handles two main situations: over temperature protection and temperature control. Over temperature protection reduces the incidence of abnormally high temperatures. Its job is to isolate the switch before temperatures exceed the critical safety point. Temperature control refers to cycling around the heat and fan temperatures that switch these devices on and off.
The CMOS temperature switch has a basic temperature switch circuit. The switch's operation depends on the chip temperature which can change very suddenly whenever the temperature reaches a dangerous range. This is also known as the threshold program which is a programmed function.
Choose the best thermal device for the job
Each temperature switch is built to complement a specific application. At face value you may notice the variation in many applications and assume that one temperature switch is better than another. But this is not the case. Each thermal device is designed for a purpose and should be able to meet a wide range of criteria.
It is important to compare and contrast the varying technology that creates these devices. The simple design and engineering that allows them to operate independently is a big reason for why a temperature switch is such a necessary device.
Source
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