CEE Relays Ltd

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Learn More About Integrated Arc Detection
Some of the digital multi-function relays in CEE's NP900 range can be manufactured with built-in Arc Detection units.  This page is intended to provide an overview of how they work, what the different technologies are, and what benefits they can offer your company. 
Arc Detection Logo
What is an Arc?
Electric arcs are dangerous releases of energy which occur when an electrical fault is not properly interrupted. Most modern switchgear is designed to contain, quench or prevent an arc from forming but arcs may still occur. This may be due to improper use of switchgear, damage to part of the distribution system or overloads exceeding the system’s design current interrupting capacity.
Electric arcs are characterised by:
  • Very bright, intense light
  • Pressure waves, akin to an explosion in extreme cases
  • Intense, localised heat
  • Noises ranging from "fizzing" or "popping" sounds to deafening explosions
  • Distortion in the voltage waveform of the power supply feeding the arc
  • Excessive current draw
The magnitude and duration of these effects is dependent on how much energy flows into the arc.  This in turn is affected by the fault level and impedance of the power system and by how quickly the fault is isolated.  CEE's integrated arc detection systems aim to offer the fastest possible trip time in the event of an arc. 
Arc Detection Technologies
In theory, any of the six characteristics of electric arcs listed above could be used as a means of identifying when and where an arc is taking place.  To be absolutely certain that an arc is in progress, it may be advisable to use a combination of different types of sensor in your detection system.  The following are technologies employed by CEE's range of arc detectors for the NP900 relays:

Light sensors
Detect the light produced by an arc fault.  They are in the form of point-sensors to detect light in a specific place within a switchgear compartment.
These sensors are calibrated to only send a "trip" signal when a certain minimum intensity of light has been detected.  They are non-latching. See the "Arc Detection Modules and Accessories" section below for the available thresholds.
Light sensors are the primary arc detection mechanism used by NP900 relays.

Pressure sensors
Detect the pressure wave produced during an arc. The type supplied by CEE are always combined with a light sensor such that the sensor will only send a "trip" signal if it detects both light and overpressure at the same time. Pressure sensors can be a good way of reducing spurious trips, since there is rarely any source of pressure within the switchgear other than an arc.  The exception would be SF6 gas-insulated switchgear; in this instance the equipment should be isolated upon loss of gas pressure anyway. 
The detection time of pressure sensors depends heavily on the mechanical design of the switchgear compartment being protected.  For a small compartment, using pressure-and-light sensors typically adds 5ms to the arc detection time over and above the response time of a light-only sensor.  Pressure sensors are not considered appropriate for large, unsegregated switchgear panels or for compartments which are not totally enclosed. 

Current sensors
NP900 relays fitted with Arc Detection units do not employ dedicated current sensors for detecting arcs.  Instead, those relays with inbuilt overcurrent protection can be programmed to ignore inputs from light sensors (or light-and-pressure sensors) unless they detect a simultaneous increase in the current drawn. This helps to avoid spurious trips caused by light from other sources (inspection lamps, camera flashes or brief arcs across contactor terminals).  It can also be used as a form of discrimination; if several current sources pass through a compartment where light is detected, only those experiencing high currents will be isolated. Since the relay must detect both light and overcurrent at the same time, the operating time will be slightly longer (see the "Benefits of Arc Detection" section below for details).
Two overcurrent settings are available in the arc detection module: phase overcurrent and residual overcurrent. These may be set to different thresholds as appropriate.
Arc Detection Modules and Accessories
Selected NP900 relays can be specified with an Arc Detection module which has 4 sensor input channels.  Up to three sensors can be connected in parallel, allowing the module to accept a maximum of 12 sensors in total. The four different channels allow you to build a system with up to four different zones of protection if required. 
Below are some details about the individual arc detection components.  Arc detection schemes which incorporate overcurrent measurements will use the NP900's CTs (where applicable).  However, overcurrent settings associated with arc detection are independent of any overcurrent-only settings (instantaneous, definite time or IDMT) programmed into the relay. 
Arc Detection Module
Sensor inputs
4 (twisted pairs) May be used to connect light sensors or combined light-and-pressure sensors
Binary inputs 1 (24VDC) Semiconductor input, may be used for discrimination with other arc detection modules
Binary outputs 2 (normally open) High-speed semiconductor outputs (250VDC withstand, 2A continuous carry). It is recommended that these are used for signalling only; the NP900 trip output should be used to initiate trips.
Light Sensors
Light detection radius
180o Sensor should be mounted facing the most likely source of an arc (terminals and cable joints are recommended).
Maximum cable length 200m Maximum cable length per sensor. Sensors connected in parallel across the same Arc Detection input can each have cables up to 200m long. 
Light intensity threshold (A) 8000 lux Option A.  This approximates to the light intensity of a phase-ground arc on a 200A system.  
Light intensity threshold (B) 25000 lux Option B
Light intensity threshold (C) 50000 lux Option C
Light sensors have normally open semiconductor outputs which are non-latching.  However, when correctly connected to the Arc Detection module on an NP900 relay, the NP900 will store a record of any light detection events in non-volatile memory.
Light and Pressure Sensors
Light detection radius
180o Sensor should be mounted facing the most likely source of an arc (terminals and cable joints are recommended).
Maximum cable length 200m Maximum cable length per sensor. Sensors connected in parallel across the same Arc Detection input can each have cables up to 200m long. 
Light intensity threshold (A) 8000 lux Option A.  This approximates to the light intensity of a phase-ground arc on a 200A system.  
Light intensity threshold (B) 25000 lux Option B
Light intensity threshold (C) 50000 lux Option C
Combined sensors have normally open semiconductor outputs which are non-latching.  They must detect both a sufficiently strong light source and a sudden increase in pressure at the same time before sending a signal to the NP900 relay
Benefits of Arc Detection
Arc detection systems can react much faster than traditional overcurrent protection systems since they do not need to discriminate with upstream protective devices or allow short-duration overcurrent situations such as motor starts or energising transformers.  Put simply, intense lights or pressure waves within switchgear are generally "bad" situations requiring an immediate trip.  
Very short trip times are useful since they limit the amount of energy released during an arc. Typical trip times for arc detection on NP900 relays are shown below (these exclude circuit breaker operating times, which will depend upon the make and model):
  • Light only typically 11ms (up to 18ms max)
  • Light and overcurrent 17ms (up to 22.5ms max; assumes overcurrent has been set to the recommended threshold of 1.5x nominal phase current)
  • If light-and-pressure sensors are used instead, trip times listed above will increase by about 5ms for a small switchgear compartment.  Sensors in larger compartments could take longer to detect an overpressure event, depending on the relative positions of the arc and the sensor.
Are you unsure of the potential arc energy that could be released by your system?  Consider the Power*Tools for Windows modelling software or ask CEE for an arc protection study