Arc Suppression Factor
Arc suppression methods and devices may be compared using an Arc Suppression Factor (ASF), measuring the ratio of the arc energy without suppression (Wno a/s) over the arc energy with suppression (Wa/s) as follows:
ASF = Wno a/s / Wa/s
The ASF allows arc suppression devices to be objectively compared as follows:
ASF = 1 ……… No Effect
ASF > 1 ……… Positive Effect (i.e., arc energy is decreased)
Using the examples shown on the Suppression Measures page (here), based on the findings in Lab Note 103 (here), yields the following example calculation:
AC Power ASF Calculation
AC power arc without arc suppression
The break arc lasts about 7ms as shown, until the current approaches the zero-crossing. Looking at this scope picture you can almost “feel” the arc burn through the widening contact air gap.
Warc0(unsuppressed arc energy) ≈ 437mJ
Calculation: Warc0 = Iarc0 x Varc0 x Tarc0
Mean Iarc0 ≈ 2.5A
Mean Varc0 ≈ 25V
Mean Tarc0 ≈ 7ms
AC power arc with NOsparc arc suppression
The break arc is arrested as it forms, and is suppressed approximately 7ms until the current approaches the zero- crossing. We refer to this initially arrested arc as an “arclet.”
Warclet(arclet energy**) ≈ 350µJ
Calculation: Wa/s = Ia/s x Va/s x Ta/s
Mean Ia/s ≈ 7A
Mean Va/s ≈ 10V
Mean Ta/s ≈ 5µs
** Energy in the initial, nascent arc just prior to suppression
The ASF calculation for an arc suppressed by NOsparc technology is:
ASF ≈ 1250 … A Very Significant Effect
ASF = Warc0 ÷ Warclet
Mean Warc0 ≈ 437mJ; Mean Warclet ≈ 350μJ
Electrical arcing across the contacts of an electromechanical relay may be effectively measured using an oscilloscope connected to a differential voltage probe across the relay contacts and a high speed current probe to measure the current through the contacts during operation under load.
The results obtained using this test set-up allow for determining the effectiveness of a contact arc suppression on either an electromechanical relay or a contactor. This is achieved by simultaneously measuring the current through the contact and the voltage across the contact during either the MAKE or BREAK transitions. This test requires both a high-speed current probe and a high-voltage differential voltage probe, as well as a two-channel, externally and delay triggered oscilloscope. The current probe must be connected to measure the current through the contact, with its output signal fed into the oscilloscope’s channel 1. The voltage probe must be connected across the contacts, with its output signal fed into the oscilloscope’s channel 2. The relay driver’s signal should be fed into the oscilloscope’s external trigger input, and a delayed trigger employed to bring the instant of contact transition into view on the scope. Triggering on the rising edge will display the MAKE transition, while triggering on the falling edge will display the BREAK transition.
The figure below-left is a diagram of a typical contact protection arrangement without arc suppression. The figure below-center is a diagram of a typical contact protection arrangement with arc suppression. The box labeled “Load Switching Control Device” in each figure represents a process control relay whose arc is being measured.
The Four Relay Contact States:
2. BREAK (transition state from closed to open)
4. MAKE (transition state with “bounce” from open to closed state)
The relay contacts move from open to closed and back in four distinct states shown in the figure above-right.ing.