How Spark Gap SPDs are used for Testing and Selection of Voltage Protection Level & Follow Current

SPD is a device designed to protect electrical circuitry from voltage spikes. SPD limits the over voltage in an electrical network by diverting, blocking or by shorting any unwanted voltages above a safe threshold. This limiting is voltage also known as let-through voltage, voltage protection Level, spark over voltage, clamping YROWDJH HWF., VSHFLÀHV WKH RYHU YROWDJH ZKLFK FDXVH
the protective components inside a surge protector to
divert unwanted energy from the protected line. A lower clamping voltage indicates better protection.
Surge protectors don’t operate instantaneously, a slight delay exists. The longer the response time, the longer the connected equipment will be exposed to the surge. However, surges don’t happen instantly either. Surges usually take around a few microseconds to reach their peak voltage, and an SPD with a nanosecond response time would kick in fast enough to suppress the most damaging portion of the spike.
A spark gap arrester consists of an arrangement of two
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with a gas such as air, designed to allow an electric spark to pass between the conductors. When the voltage difference between the conductors exceeds the breakdown voltage of the gas within the gap, a spark forms, ionizing the gas and drastically reducing its HOHFWULFDO UHVLVWDQFH. &XUUHQW WKHQ ÁRZV XQWLO WKH SDWK RI
ionized gas is broken or the current reduces below a
minimum value called the “holding current”.
Spark gap is one of the oldest protective device, having been developed in the nineteenth century for use in telephone lines. During this time, a carbon rod electrode LV KHOG ZLWK DQ LQVXODWRU DW D VSHFLÀF GLVWDQFH IURP D
second electrode. The gap dimension determines the voltage at which a spark will jump between the two parts and short to ground. Carbon block spark gap over voltage suppressors are similar to gas discharge tubes (GDTs) but with the two electrodes exposed to the air, so their behavior is affected by the surrounding atmosphere,especially the humidity. Since their operation produces an open spark, these devices should never be installed where an explosive atmosphere may develop.
Spark gaps were used historically in early electrical equipment, such as spark gap radio transmitters, electrostatic machines and X-ray machines. Their most widespread use today is in spark plugs to ignite the fuel in a petrol engines. They are also used as lightning arrester in power line in different forms such as Rod Gap Arrester, Horn Gap Arrester, Multi Gap arrester, Expulsion type arrester, Valve type arrester, Externally gapped line arrester etc., to protect electrical network and transformers from high-voltage transients.
Spark gaps are widely used as Class 1 SPD in LV power lines. These are called as Voltage switching type SPD’s. The advantage of the device is its high energy handling capacity to the order of up to 100 KA (10/350 μS). The disadvantage is more response time and the chance of follow current compared to other protective devices such as MOV’s and SAD’s.

 

IEC 61643 is the standard by which the performance requirement and testing methods of SPD’s are tested. Part 1, Edition 1 was published during 1998 which was replaced by edition 2 in 2005 and then by part 11 edition 1 of 2011. Major improvements are from Performance tests to safety and performance tests.

Spark Over voltage of a spark gap SPD is the voltage it allows in the circuit during a transient. This test is carried with an impulse voltage of 1.2/50 µS waveform with an open circuit voltage of 6 KV. 10 impulses are applied to

 

voltage at the SPD is recorded with an oscilloscope. In 1998 version of IEC 61643, the measured limiting voltage is the average of 10 measured peak values. Whereas  in the improved versions, maximum spark over voltage measured is considered as limiting voltage.

Spark over voltage varies after every test pulse. If 9 tests resulted in a spark over voltage of 1.2 KV and the balance one recorded 4 KV, according to the 1998 version, the spark over voltage is 1.25 KV and in 2011 version the spark over voltage is 4 KV. With a real lightning impulse current, further chances of deteriorations are expected,

 

electronic equipment.

Another important problem in a spark gap  arrester  used in a 230/400 volt power line application is its  follow current. This is applicable for Spark Gap SPD’s connected between L and N or L and PE. The supply

 

the spark gap based SPD’s, which is called as follow current. This Follow current  causes  additional  stress in the upstream devices, wiring etc and may cause un expected failures, if not designed and installed properly.

Follow current is nothing but the short circuit current from the network which equal to the short circuit capacity of the network. SPD’s need to not only need to extinguish follow current with out the help of an additional back up fuse, but withstand the high arcs created. This makes the Spark Gap SPD, a critical component in high capacity Power Panels. Generally these SPD’s are used with a HRC backup fuse. At a critical condition, the backup fuse

 

SPD at an appropriate time.

Conclusion

Small change in the test parameter of a standard creates

device. There are chances where manufacturers provide misleading  information  and  compliances.  The easiest

spark over voltages are similar, it is better to cross check the latest test reports from an international laboratory such as VDE, KEMA, OVE etc.

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