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High Performance Current Sensing for Automation

High Performance
Current Sensing for Automation

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Equipment Ground Fault Monitoring — Marinas

Ground Fault Detection in MarinasSafety is essential whether in the workplace or during recreational activities to avoid the possibility of electrical shock.  Boat docks and marinas are places where work and recreational activities occur in the same place. Additions to the National Electric Code (NEC) section 555.3 stipulate the electrical system powering marinas be protected with ground fault detection and interruption. The entire power supplied to docks and boats can be monitored for ground leakage current or each branch circuit can be monitored independently. If there is a fault of 100mA or higher, the power must be disconnected from the source. This low level fault current is generally not enough to trip the circuit breaker protecting the circuit or to open a fuse, but can cause a shock or can damage equipment.

While monitoring the main power would be effective, a fault would cause the main to disconnect all of the power to the marina when there is minimal fault at several of the connected loads. A better approach to detecting fault current so that the majority of the system remains operational is to use fault detection at each power pedestal.

With several boats connected to separate power pedestals, each boat could have small amounts of current flowing to earth ground. Bilge pumps, fans and transformers are common causes of low level fault leakage to earth as the insulation over the windings can develop small cracks. The moisture in the environment helps to make an electrical path to the equipment earth bond. Incorrect or damaged wiring is also common, and electric water heating elements can and do fail, shorting to ground.

AG sensors have an aperture of 0.74 inches, and AGL have an aperture of 1.88 inches. To ensure the wires fit, measure the OD of one wire. Multiply by 2.15 for the OD of a bundle of three wires. Multiply by 2.41 for the OD of a bundle of four wires.

AG sensors have an aperture of 0.74 inches, and AGL have an aperture of 1.88 inches. To ensure the wires fit, measure the OD of one wire. Multiply by 2.15 for the OD of a bundle of three wires. Multiply by 2.41 for the OD of a bundle of four wires.

Each of these relatively small amounts of fault current will sum together at the main circuit protection and can easily cause “spurious” tripping. The ground fault sensor detects all fault current downstream from the point where it is installed. If five boats each have low level leakage to earth, say 20mA, the total leakage sensed at the main circuit breaker would be 100mA, and that would trip the ground fault sensor.
Protecting the power to each boat with a circuit breaker equipped with shunt trip mechanism is a better solution. An external fault sensing device surrounds the wires to the power receptacles.

When a fault is detected the sensor closes a circuit to operate the shunt trip mechanism, shutting off power to the single pedestal and the boat or the connected equipment with the problem. The power is restored by pushing the breaker handle back to the closed condition after the problem is identified and corrected.

A contactor can also be used to disconnect the load, although we would recommend selecting a sensor with a latching output. this type of contact action, the power to the operating coil of the contactor is de-energized after a fault is detected and the contactor interrupts the faulting circuit. The sensor is reset by a manual action after the fault is remediated.

Most marinas provide 120/240 VAC single phase power to the docks. The power supplied to each pedestal is usually between 50 and 100 amps. When selecting the proper sensor, be certain to choose one with a window large enough to allow all the current carrying conductors to pass through. The AG Series sensor is large enough to allow up to three #6 THHN to be threaded through the window. The AGL Series can accommodate up to three 300 MCM THHN. The bonding ground wire is not passed through the sensor. The wire feeding the pedestals may be larger than what might be seen in other installations as the distance from the main panel to the pedestals is significant. The wire gauge must be increased to overcome voltage drop. This reduction in voltage should be limited to 5% of the primary circuit voltage.

NK Technologies’ Ground Fault Sensors

ag-aglSince the late 1960’s, the National Electric Code has required ground fault circuit protection in many areas; each revision cycle seems to add new requirements More recent versions have added requirements for equipment protection, which are designed to prevent damage from phase to earth faults. Monitoring a three phase circuit for ground fault current requires more than comparing the hot and neutral loads. NKTechnologies’ sensors monitor all current carrying conductors at the same time. If more current is used at the load than is being returned to the source, there is earth leakage. This leakage can be detected by using our extremely sensitive ground fault detection products.

Related Products

Power Factor Correction Control

With ever increasing demands for electrical power the power generating Power Factor Correction Controlcompanies are beginning to penalize customers with low Power Factors. Low PFs can be considered very inefficient use of electrical energy. A low PF can be corrected by the installation of PF Correction capacitors. Although this can be achieved at the main feeder most facilities, due to multiple varying loads, choose to install PF correction on individual pieces of equipment (like a large motor) or area of plant. When a load is switched off, unless the PF correction capacitors are removed, undesirable over-correction occurs. Sophisticated automatic control systems are available but are very expensive. A simple way to switch a PF correction capacitor bank is to use an NK Technologies’ Current Operated Switch. When the load goes down, for example at night when equipment is not used, the PF correction capacitor is switched out. Then when load returns it is switched back in.

Power Transformer Monitoring

Installing an NK Technologies’ Current Transducer over the conductors on the Power Transfomer Monitoringsecondary of a transformer can be used to alarm if the current draw reaches a critical level. Action can then be taken to reduce the load before the transformer overheats.

Most large power transformers utilize auxiliary cooling fans, drawing cooler air from a distance away from the transformer. NK Technologies’ Current Transducers or Current Operated Switches are used to monitor these fans for overload from bearing failure, under loads from drive component breakage or slippage, and to just be sure that the fan is energized and doing its required job.

Equipment Ground Fault Monitoring

As we become more aware of occupational health and safety so the need for Equipment Ground Fault Monitoringprotection against ground faults is growing. Industrial electrical equipment is fed via a distribution system which in turn is connected to a main feed. Ground fault protection may be installed, for instance at a main breaker, protecting the branch circuit but not necessarily a valuable or critical piece of equipment. Industrial Ground Fault Sensors from NK Technologies are designed to be installed in individual pieces of equipment or their electrical feed. The sensors provide an on/off output that may be used to operate a circuit interruption device like a shunt trip breaker or feed into an alarm system. With over 100 models to choose from, there’s sure to be one to fit your application.

Battery Charging System

Industrial re-chargeable battery systems are used to provide primary power for Battery Charging Systemvehicles and back-up systems for critical equipment, like cell phone base stations and other communications equipment. DC current flows in one direction during charging and in the opposite direction during battery operation. Traditional Current Transformers do not work with DC current and using current shunts has been the solution. The shunt operates by producing a voltage potential across its terminals according to Ohm’s law. It suffers from a number of serious disadvantages:

  • Terminals can rise to the potential of the DC circuit.
  • The signal produced is usually 50 – 100mV at full rated current and must be isolated and amplified to a higher level signal (typically 4-20mA) to be of use to an industrial monitoring or control system.
  • The shunt dissipates power according to the square of the current flowing through it. This manifests itself as a loss of power or an insertion loss.
    Forget the shunt and instead simply pass the current carrying conductor through the aperture of an NK Technologies’ DC Current Transducer. There is no insertion loss and the 4-20mA signal is fully isolated from both the DC current being measured and the power supply. The bipolar option also accommodates the change in direction of current flow.

Generator Installations

The need for back-up power for critical loads is becoming more and more Generator Installationsapparent to operating and maintenance personnel. Many existing factories and facilities are also adding generators to assist the grid connection supply during times of excessive consumption, or to help the plant ride through periods of peak usage when the utility charges a premium for power. Monitoring the current demand provided from a generator allows for load shedding of non-critical loads when the generator is nearing its capacity. NK Technologies’ Current Operated Switches close when the demand, measured by current, is too high, and by connecting to a controller (like a PLC), less critical loads can be disconnected from the system.

Current Transformer Monitoring

C.T.s (Current Transformers) have been used for many years to measure AC Current Transformer Monitoringamperage. There are many C.T.s installed throughout Electric Utilities, Process Plants and Manufacturing facilities. Typically a C.T. provides either a 1A, or more commonly a 5 Amp AC output on the secondary side. In order to use this signal for a modern control system (DCS, PLC or DDC) or Data Logging System a signal conversion or conditioner device is required to provide an analog DC signal.

C.T.s step down current and the secondary has to feed into a very low impedance to maintain accuracy. In the event that the secondary is open-circuited during operation the device operates as a step-up voltage transformer since there is nowhere for the current to flow and voltages of several kV appear on the secondary, overheating occurs and combustion or explosion often results. NK Technologies’ Split-Core Current Transducers are simply and safely snapped over the shorted secondary of a C.T. to provide a fully isolated analog 4-20mA DC signal proportional to the current in the secondary, which in turn is proportional to the measured (primary) current.

NK’s Split-core Current Transducers are used to safely monitor the secondary of high voltage C.T.s used in power generation and distribution in the low voltage cubicles.

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