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Induction Electrical System Safety When melt shops relied on fuel-fired furnaces, the electrical hazards in foundries were similar to those in any other industry. Electrical motors, lift-truck battery chargers, heaters, lights and office equipment operated at standard voltages. The switches, connectors and circuit interrupters encountered at work were simply larger versions of those found at home. Like all industrial workers, foundry workers realized the need to treat electricity as a force to be respected. But at the same time, their years of experience in living with electrical devices taught them electrical hazards could be easily avoided.
The introduction of induction furnaces made it necessary for foundry workers to work in close proximity to high voltage power supplies and open air-cooled bus bars, apparatus commonly associated with dangerous power company substations. They also had to learn that a certain amount of sparking and arcing between the metal pieces in a cold charge is normal in an induction melting furnace and not necessarily a sign of an imminent catastrophe. While induction systems present more exposed conductive surfaces than other industrial equipment, they are designed with a variety of safety systems to deal with these hazards. For example, current handling bus bars and components are surrounded by enclosures. Safety interlocks turn off power if power supply access panels or doors are opened while the unit is running. They are also designed to prevent accidental starting if access panels or doors are open.
If your power unit does not have safety locks and interlocks on all doors and access panels, it should be modified to add these devices. Your equipment manufacturer should be able to assist you in adding these important safety devices. Cabinet door locks are the most important barrier to unauthorized access to the dangerous electrical elements inside the power cabinets. These doors must be kept locked at all times.
High-current bus bar is encased in clear plastic to prevent accidental contact. This power cabinet door latching bar provides both a mechanical barrier to door opening and an electrical interlock. ________________________________________________ Safety Recommendations for Foundry Supervisors & Managers Supervisors need to be especially aware of electrical safety. Increased use of induction furnace technology has made it necessary for a growing number of maintenance and repair workers to come into close proximity to high current conductors. Many maintenance technicians, particularly those who work with low-voltage devices, such as control systems, do not fully appreciate the risk posed by the high levels of voltage and current used in induction melting. It is imperative these individuals be impressed with the fact that shortcuts, such as overriding safety interlocks during troubleshooting, are absolutely unacceptable when working with even the smallest induction furnaces and power supplies. Only fully-trained personnel are to have access to high-risk areas. A safety lockout system is another effective measure to prevent electrical shock. ________________________________________________ Safety Lockouts are Key Systems
The safest way to prevent power being turned on accidentally while equipment is being serviced is with a safety lockout system. With this system, the individual performing the service work uses a lock to secure the circuit interrupter in the off position. He or she keeps the only key until the service work is complete and the equipment is ready to be restored to operation. At that time, the same individual who put on the lock, removes it, allowing the breaker to be closed and power to be turned on.
________________________________________________ Induction Power Units Include Some or All of These Safety Systems Safety Interlocks - Safety Interlocks are designed to turn off power automatically when power cabinet service access doors are opened. Equipment must not be operated unless all interlocks are in proper working order. System Self Diagnostics - System Self Diagnostics in many advanced induction power systems prevent the unit from operating when a fault is detected and identify the location of the fault. Ground Leak Detector Systems - These systems are crucial. They turn off power if metal in the furnace comes close to or touches the induction coil or if inverter output is otherwise grounded. Battery-Operated DC Pump - A battery-operated DC pump provides emergency cooling water to your furnace if normal power is interrupted. Ultra-Fast Acting AC Interrupter Module - This serves as a solid-state circuit interrupter and expedites the system’s response to emergency conditions. Current Limiting Reactors and Quick-Acting Circuit Interrupter - These provide protection against component failure and major line disturbances on smaller systems.
Capacitor Pressure Switches - These help prevent pressure buildup inside the capacitor case by shutting down the power supply if pressure builds up due to capacitor malfunctions. If this pressure buildup is undetected and power remains on, the capacitor will explode. Line Isolation - All induction furnaces need to be designed so that the current flow from the output circuit or from output components external to the converting device to ground under operating and ground fault conditions shall be limited to a value that does not cause 50 volts or more to ground to appear on any accessible part or the heating equipment and its load. This protection may be provided through an isolation transformer located either between the inverter and the furnace (secondary isolation) or between the incoming power line and the inverter (primary isolation).
________________________________________________ For Trained Electrical Technicians Only
Obviously there will be times when electrical measurements need to be taken from energized circuits. This work must only be done by qualified electricians. Any manufacturers’ manuals, circuit diagrams or drawings that are used to guide the work must be double-checked to be sure they are complete and up to date. Before performing a test on an energized circuit, the technician must verify that he has selected the appropriate measuring instruments rated for proper voltage and current and fully understands the manufacturer’s directions. Power cords and test leads must be inspected and, if necessary, replaced with parts recommended by the instrument supplier. The rating of the measuring instruments must be higher than the electrical parameters of the electrical equipment. Power supply settings must never be set to exceed the capacity of the instruments or the test leads. Test instruments must be properly fused and grounded. Before the technician enters the energized areas, power sources and current paths should be clearly identified. The circuit must be turned off and locked out until test instruments are properly set and leads connected. Technicians must never touch leads, instruments, or settings while the circuit is energized. Power must be turned off and capacitors fully discharged before changing instrument settings or disconnecting leads from the system. If resistance measurements are part of the test program, power must be disconnected and all capacitors fully discharged before they are performed. Technicians conducting tests near energized circuits must be dressed in appropriate Personal Protective Equipment (PPE), including dry, insulated gloves as required by governing codes (i.e., NFPA 70E). They must stand on a dry, insulated surface capable of withstanding the voltages that may be encountered. The floor beneath the insulated surface must be dry, as must the technicians’ hands and shoes. A supervisor must not allow technicians performing tests to work alone. If he/she cannot be personally present during the test, he alerts nearby personnel to the nature of the work taking place and instructs them how to react in an emergency. After the job is complete, temporary grounds and bypass connections are removed and covers, guards and fuses replaced. Supervisors must verify that all safety devices and interlocks are fully operational. If modifications have been made to the equipment, the appropriate changes immediately must be made to equipment manuals, diagrams and drawings. The reason for the alteration, person making the changes and the person under whose authority they were acting should be noted; along with the time and date the modifications were completed. Everyone, including subcontractors and other off-site personnel, having copies of the original equipment manuals or drawings must be promptly provided with copies of the updated documentation. ________________________________________________ Common Electrical Regulatory Notices & Requirements NFPA 70E - 110.3 - “The safety related work practices shall be implemented by employees. The employer shall provide safety related work practices and shall train the employee who shall then implement the training.” OSHA 1910.303 (g) (2) NEC 110.27 (A) - “Live parts of electric equipment operating at 50 volts or more shall be guarded against accidental contact by approved cabinets or other forms of approved enclosures, or by any of the following means:” OSHA 1910.333 (a) (1) - “Live parts to which an employee may be exposed shall be de-energized before the employee works on or near them, unless the employer can demonstrate that de-energizing introduces additional or increased hazards or is infeasible due to equipment design or operational limitations. Live parts that operate at less than 50 volts to ground need not be de-energized if there will be no increased exposure to electrical burns or to explosion due to electric arcs.” OSHA 1910.333 (6) (2) (ii) © - “Stored electric energy which might endanger personnel shall be released. Capacitors shall be discharged and high capacitance elements shall be short-circuited and grounded, if the stored electric energy might endanger personnel.” OSHA 1910.333 (2) (IV) (B) - “A qualified person shall use test equipment to test the circuit elements and electrical parts of equipment to which employees will be exposed and shall verify that the circuit elements and equipment parts are de-energized. The test shall also determine if any energized condition exists as a result of inadvertently induced voltage or unrelated voltage back-feed even though specific parts of the circuit have been de-energized and presumed to be safe. If the circuit to be tested is over 600 volts, nominal, the test equipment shall be checked for proper operation immediately before and immediately after this test.” OSHA 1910.334 © (3) - “Test instruments and equipment and their accessories shall be rated for the circuits and equipment to which they will be connected and shall be designed for the environment in which they will be used.” OSHA 1910.306 (g) (2) (iii) - “Where doors are used for access to voltages from 500 to 1000 volts AC or DC, either door locks or interlocks shall be provided. Where doors are used for access to voltages of over 1000 volts AC or DC, either mechanical lockouts with a disconnecting means to prevent access until voltage is removed from the cubicle, or both door interlocking and mechanical door locks, shall be provided.” OSHA 1910.306 (g) (2) (iv) - “‘Danger’ labels shall be attached on the equipment and shall be plainly visible even when doors are open or panels are removed from compartments containing voltages of over 250 volts AC or DC.” IMPORTANT: |
