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Advanced Protection Technologies
Surge Protection Overview
Prepared by: Lou Farquhar, PE, CEM, GBE
VP – APT Engineering Service
(800) 237-4567
• 26 Years Supplying Surge Protective
• ISO 9001-2008 Quality Management System
• ISO 17025 evaluation by UL
• Power Quality Assurance magazine –
PQ 50 Company
• Frost & Sullivan ranked APT –
Leading Supplier of Three Phase Devices
• Market Engineering Customer Focus
• Members of UL, IEEE, NEMA standards
Copyright APT 2012
What Is a Surge/Transient?
• High amplitude, short duration overvoltage
• Can be positive or negative polarity
Transient: > two times system’s RMS voltage
< two times system’s RMS voltage
Transient Overvoltage –
Can be thousands of volts
Millionths of second
Copyright APT 2012
What Causes Surges/Transients?
• Lightning
• Switching:
– Load Switching – utility & customer
• Motors, Large Loads, Faults,
Fuse Operation
– Source Switching
• Smart Grid, Gensets, PV, Wind
• Internally generated surges: ≈70%
• Externally generated surges: ≈30%
Copyright APT 2012
In outdoor environment,
this ratio probably reverses
Effects of Transient Voltages?
• Microelectronics Intolerant to Surges
• Disruption
– Lockups, Downtime & Interruption costs
– Computing glitches and errors
• Degradation
– Microelectronics
– Slow & continuous damage to motor insulation
• Destruction
– Failed microelectronics, ballasts,
motors, controllers, etc.
Maybe analogous to:
- ‘Water hammer’ in a plumbing system
- ‘Rust’ to microelectronics
Copyright APT 2012
MOV - Metal Oxide Varistor
Varistor - variable resistor
Semiconductor; generally zinc oxide
Connects parallel to load (not series)
Thickness determines clamping voltage
Diameter determines current capacity
Copyright APT 2012
MOV symbol
MOV - Metal Oxide Varistor
MOV seeks to equalize overvoltage
Voltage sensitive conductor: V = IR & I = V/R
At ‘low’ voltages: very high impedance, 109: I  0A
Above ‘threshold’ voltage: resistance approaches 0: I = high A
– Current diverts through MOV as I = V/R (high V, low R)
Normal voltage
V 120V
= 0.12A
R 10 
V 6000V
= 6000A
R 1
Trivial leakage current
Surge Current
Copyright APT 2012
MOV - Metal Oxide Varistor
• Overvoltage diverts through MOV as current
• Voltage is “clamped” or “equalized” as energy is transferred to
other side of MOV(s)
• MOV does not ‘absorb’ surge, however, I2R heat is retained
• Bidirectional – Operates same for positive or negative surges
• Creates a momentary low impedance (acting like short-circuit)
to pass transient energy to earth; analogous to water heater
pressure relief valve
Copyright APT 2012
SPD Operation
Load 1
Load 2
Load 3
MOV/SPD Acts as a momentary ‘short circuit’
‘short circuit’ ≈ no overvoltage ≈ protected load
After the surge, MOV/SPD automatically resets
itself to high impedance state drawing no current
Copyright APT 2012
Layman’s Terms: Visualize an ‘electronic guillotine’
that chops the head off a surge and sends it away
SPD Operation
Load 1
Load 2
Load 3
MOV/SPD Acts as a momentary ‘short circuit’, then
resets itself after the surge
Copyright APT 2012
‘short circuit’ ≈ no overvoltage ≈ protected load
SPD Connector Leads
• Need short lead lengths!
• NEC 285.12: “The conductors used to connect the
SPD (surge arrester or TVSS) to the line or bus and
to ground shall not be any longer than necessary
and shall avoid unnecessary bends”
• Industry typically states: Each foot of conductor
adds 100 - 170V to clamping voltage
• No Sharp bends or kinks
• No Wire Nuts!
• Right Hand Rule – can cancel inductive effects by
bundling, tie-wrapping conductors together
Copyright APT 2012
2008 NEC Art 285
Copyright APT 2012
2011 NEC Art 285
Copyright APT 2012
APT Here to Help:
(800) 237-4567
Copyright APT 2012
Specification Assistance
Sounding Board for issues
Competitive crosses or analysis
General Help
On-Line webinar services
Forensic Testing & Analysis of failed SPDs