Over the years Underwriters Laboratories (UL) 1449, Institute of Electrical and Electronics Engineers (IEEE) standard C62.41, and International Electrotechnical Commission (IEC) 61643-1 surge protection terminology has been changing.
These changes and different marketing campaigns have added confusions when a layman needs to of select the correct Ac SPDs.
Example: No Nationally Recognized Testing Laboratory (NRTL) recognizes a Joule rating as an accurate rating of performance of Ac SPD. The acceptable is the kA rating per mode
Selecting the correct Ac SPDs involves more than just pricing. Ac SPDs needs to be chosen for the level of protection required for the equipment being protected. If not, then the Ac SPDs that are installed could be worthless from an equipment protection point of view.
Need To Know Terminology For Ac SPDs
Note 1: IEEE Uses Categories instead of Types
Category C: Service Entrance, more severe environment: l0kV, l0kA surge
Category B: Downstream, ≥ 30' from category C, less severe environment: 6kV, 3kA surge
Category A: Further downstream, ≥ 60' from category C, least severe environment: 6kV, 0.5kA surge
Note 2: IEC Uses Type and Class instead of just Types
Type 1 / Class I / Class B
Type 1+2 / Class I+II / Class B+CType 2 / Class II / Class C
Type 2+3 / Class II+III / Class C+DType 3 / Class III / Class D
Terminology Content For Ac SPDs
Modes Of Protection: Every Ac SPD made does not have each mode of protection protected.
Voltage Protection Rating (VPR): The VPR is the voltage value where the Ac SPD limits a specific surge between modes of protection. The lower the VPR the better the equipment is protected
When the SPD test results are:
The SPD VPR is:
330 Volts ≤ 330
331 Volts ≤ 400
401 Volts ≤ 500
501 Volts ≤ 600
601 Volts ≤ 700
701 Volts ≤ 800
801 Volts ≤ 900
901 Volts ≤ 1000
1001 Volts ≤ 1200
1201 Volts ≤ 1500
1501 Volts ≤ 1800
1801 Volts ≤ 2000
2001 Volts ≤ 2500
2501 Volts ≤ 3000
3001 Volts ≤ 4000
4001 Volts ≤ 5000
5000 Volts ≤ 6000
Per UL 1449 standard each Ac SPD VPR is established from the test results using only a 6” lead.
Depending on the voltage level, the rise time of the voltage, and the frequency of the voltage each foot of cable past the 6” lead length could add between 100-200 volts per foot above the UL VPR of the Ac SPD.
Example: customer purchases an Ac SPD with a 330 VPR but the Ac SPD lead length is 36”. This means during a power surge the 330 VPR rated Ac SPD could turn out to actually have a 600 to 900 VPR. See chart below for equipment damaged time from a quick voltage surge.
Quick Surge Duration Time and PEAK Voltages That Will Result in Equipment Damage
Line Voltage (RMS/Peak)
8-20 μs Microsecond
Up to 100 μs Microsecond
More than 100 μs Microsecond
480 RMS/678 Peak
77 RMS/392 Peak
240 Volts/339 Peak
120 Volts/170 Peak
Nominal Voltage: Is the normal voltage rating of the SPD. Some common voltage examples are:
MCOV Maximum Continuous Operating Voltage: The maximum designated root-mean-square (rms) value of the power frequency voltage that may be continuously applied to the mode of protection of an SPD.
Nominal Discharge Current (In): The manufacturer shall specify (declare) the value of the Nominal Discharge Current (In) to which the sample will be tested. The value of the Nominal Discharge Current (In) selected by the manufacturer shall be: 10 kA or 20 kA for Type 1 SPDs and 3 kA, 5 kA, 10 kA or 20 kA for Type 2 SPDs. With the SPD in the circuit the surge generator voltage shall be adjusted to ensure that the value of In (selected by the manufacturer) is impressed through the SPD. The resultant waveshape is permitted to deviate from the initial short circuit calibration of the 8 x 20 μs waveshape providing that the front time is a maximum of 9 μs and the duration is a minimum of 16 μs.
Per Mode Rating Defined: An SPDs per mode rating is based on the total amount of energy it can shunt from one circuit conductor to another. If an MOV capable of shunting a 50 kA of current is used in each pathway, then the per mode rating of the SPD is 50kA.
Ac SPD Response Time: UL, IEEE and IEC have not chosen to endorse response time as a valid Ac SPD rating parameter.
Coordinating Cascaded Ac SPDs: Coordinating cascaded is stepping down the Ac SPDs VPR and kA ratings downstream to the protected equipment. However, each Ac SPD should be a minimum of 30 wire feet from each other. Some SPD manufacturers recommend what they called “a 3, 2, 1 rule of thumb” for sizing SPD’s. 300kA rating for main panel, 200kA rating for distribution panel, and 100kA for final equipment panel.