Selection of Surge Protective Device (SPD)- (Part 2)

SELECTION OF SURGE PROTECTIVE DEVICE (SPD)- (PART 2)

SIZE OF SURGE PROTECTION DEVICE (SPD) DEPENDS UPON LOCATION OF PANEL:

Panel location within the electrical system is more important than the panel’s size. * The location of the panel within the facility is much more important. IEEE C62.41.2 defines the types of expected surges within a facility as: * Category C: Service Entrance, more severe environment: 10kV, 10kA surge * Category B: Downstream more than 30feet from category C, less severe environment: 6kV, 3kA surge * Category A: Further downstream, more than 60 feet from category C, least severe environment: 6kV, 0.5kA surge * When selecting the appropriate kA rating for an SPD. * Category C: 100kA to 200kA per phase * Category B: 50kA to 100kA per phase * Category A: 50kA to 100kA per phase

LARGE SIZE OF SURGE PROTECTION DEVICE (SPD) DOES NOT GIVE BETTER PROTECTION:

Most SPDs use a metal oxide varistor (MOV) as the main limiting device. If an MOV is rated for 10kA and having a 10kA surge, it would use 100% of its capacity. The surge will degrade the MOV a little bit. * Now if we use 20KA SPD so this SPD has two 10kA MOVs in parallel. The MOVs will equally split the 10kA surge, so each would take 5kA. In this case, each MOV have only used 50% of their capacity which degrades the MOV much less than 10KA SPD * Again It is totally misleading that two parallel path (in 20KA SPD) absorb surge faster or better than single path SPD (like 10KA SPD) of same rating. * The main purpose of having MOVs in parallel is to increase the longevity or Life of the SPD. * Again, It is need to clear that it is subjective and at some point we are only adding cost by incorporating more MOV’s and receiving little benefit. * Larger kA ratings are for redundancy & longer life only.

SPD CAN NOT GIVE 100% PROTECTION AGAINST ALL TYPES OF ELECTRICAL DISTURBANCE

There is a misconception about SPDs is that they are designed to protect against all Electrical problems. * SPD is not designed to protect against excessive voltage at the fundamental power frequency. It is design to give protection against surges (by direct lighting or voltage surges in line at remote location). * SPD can not give Protection against Poor Power Quality (Harmonics) * Some SPDs contain filtering to remove high frequency noise (50 kHz to 250 kHz), But SPD cannot filter harmonic loads (3rd through 50th harmonic equals180 to 3000 Hz). * SPD can not give Protection against Under Voltage. * SPD can not give protection against under voltage problems. * SPD can not give Protection against direct lighting Strikes. * An SPD can not prevent damage caused by a direct lightning strike. A direct lightning strike causes induced surges on the power line that are reduced by the SPD But SPD can not Protect against Lighting Strikes near SPD Location. * SPD can not give protection against temporary overvoltage. * Temporary overvoltage is caused by a severe fault in the utility power or due to problems with the ground (poor or nonexistent N-G bond). * Temporary overvoltage occurs when the Voltage exceeds the nominal voltage for a short duration (millisecond to a few minutes). * If the voltage exceeds 25% of the nominal system voltage, the SPD and other loads may become damaged.

SELECTION OF SURGE PROTECTION DEVICE (SPD):

The Size, performance and specification of SPD depend on following characteristics

Current characteristic of SPD

I:Surge Current Rating (KA), * In: Nominal Discharge Current (In), * Imax: Maximum discharge Current (Imax) * Short Circuit Current Rating (SCCR).

Voltage characteristic of SPD

Uc: Maximum Continuous Operating Voltage (MCOV), * Up: Voltage Protection Rating (VPR) or surge voltage rating (SVR) or Clamping Voltage. * TOV: Temporary Over Voltage.

(1) SURGE CURRENT RATINGS (I):

The peak surge current ratings of SPD are generally based on the sum of Line-neutral and Line-ground current. * A peak ampere rating per phase. (I.e. L-N 100 kA, L-G 100 kA provides 200 kA/phase). * Other Specification like MCOV, VPR, In and SCCR that have clearly defined test criteria, but for Surge Current there is no specified Test Criteria or industry-standard hence different SPD manufacturers to create their own definitions of peak ampere surge current ratings. * Please note that selection of Higher Surge Current Ratings don’t always gives Better Protection but it is provide loner life. * IEEE Clearly states that “The selection of a surge current rating for an SPD should be matched to the expected surge environment and the expected or desired useful life of the device.” * Selection of Surge Rating for an SPD depends on The location of the SPD within the electrical distribution & environmental surroundings condition of Site. * Following surge current ratings based on SPD location within the electrical distribution.

Surge current ratings based on…