Posted on 02 May 2019

Temporary Overvoltage immunity by Surge Protection Devices

Free Bodo's Power Magazines!




Effect of temporary overvoltage (TOVs) on surge protective devices (SPDs)

If the SPD has been selected with a maximum continuous operating voltage Uc, lower than the overvoltage generated by the low voltage (LV) insulation failures or the loss of a supply conductor, the current flowing through the SPD increases very quickly and the resulting heat destroys the SPD. The effects of this failure are restricted to the SPD itself by its incorporated thermal protection. As a consequence, the installation or the equipment often remains without other overvoltage protection.

By AleÓ Štagoj, Iskra Zaščite d.o.o.

It is recognized that if the UC of the SPD is selected equal to or higher than 1.45 for TN systems and higher than √3 U0 for TT systems, a tolerable risk of loss of protection against over-voltages is achieved in all situations, except IT systems. SPD selected with a high UC (high U1mA - voltage across the varistor measured at 1 mA d. c.) will be immune to most temporary overvoltages, but at the price of diminished surge protection – higher limiting voltage (voltage protection level UP). With low UC (low varistor voltage U1mA) selected SPD we will get low limiting voltage (low UP), but at a greater risk of destruction under temporary overvoltage conditions.

Is there a solution to avoid TOV effects on SPD?

Tactic which is chosen in competitive SPDs is to use higher voltage rated SPD in order to be more resistant to TOV – but in that case UP is higher.

Iskra Zaš∨cite d. o. o. developed a new group of products, SAFETEC® family (Fig. 1), with an innovative technology, immunity to TOVs higher than 1, 5 times UC. SAFETEC® is the product with low limiting voltage (UP) which, in case of the TOV higher than its UC, will not be destroyed, as other SPDs on the market.

SAFETEC® innovative technology, patented solutions

How does SAFETEC® work on TOV?

In case of voltage increasing above the maximum value of the SPD (UC), the operating path with the current limiter (TC, Figure 2) is activated, limiting the current passing through the varistor (MOV) to the value of 10 mA. This current is below the value of the energy capability of the varistor. This means that even after the overvoltage ends, the varistor will still be functional (no need for SPD replacement!).

With our technology, we limit the current through the varistor to the value of a few 10 mA already at the very beginning, which consequently means if the thermal protection operates (disconnects), arc cannot develop or it can be broken more easily.

In cases where it comes to an overload or where the varistor is loaded beyond its capability, a release of the thermal safety occurs. The thermal safety has a patented mechanical construction, which allows the termination of the arc and permanently divides two different potentials (increase creapage distance).

Topology of SAFETEC

Behaviour of TOV tests between competitors In our laboratory we have compared SPDs´ behavior on TOV, based on IEC 61643-11:2011. We have loaded individual tested sample with 253 V, nominal a.c. r.m.s. line voltage to earth with added 10% voltage regulation (Figure 3). After 10 seconds we have switched to 438 V (previous connected voltage multiplied with √3 - TOV due to LV installation fault to earth). Both voltage sources had the capability of delivering a prospective short-circuit current of 100 A. The time interval between switching off first connected voltage and switching on second TOV voltage has not exceeded 100 ms. Through the whole test we had monitored current through the sample, measured through current clamp and voltage drop on the SPD terminals.

Test circuit to perform the comparison test, similar as in IEC 61643-11:2011, Fig. 14 (test under TOVs caused by faults in the low voltage system)

The samples for comparison test were picked up from open sales market in September 2012. We chose the 1TE DIN rail mounted pluggable products with varistor as protective element, with buildup thermal protection and declared maximum continuous operating voltage, UC closest to 275 V (Table 1).

Comparison of declared parameters IEC on four competitive products and SAFETEC

Sample No. 4 has higher max. continuous operating voltage UC and higher voltage protection level UP - due to the higher varistor voltage U1mA. SAFETEC® with UC 275 V also has higher UP – but not because of high U1mA. Varistor voltage U1mA was measured on each sample before starting TOV test (Table 2).

Test results on four competitive products and SAFETEC®

Samples have been tested according to the latest IEC 61643-11:2011 on TOV due to LV installation fault to earth. Possible real-life conditions resulted with devastating failure to the SPDs No. 1, 2 and 3 – fire and explosion.

On the other hand, sample No. 4 (with higher measured varistor voltage U1mA) disconnected after few seconds of current flowing through the SPD (Figure 4).

Oscillogram of sample No. 4, current flowing through the SPD at applied 438 V

In the case of SAFETEC® (Figure 5), when 438 V was applied on SPD, the current limiter (TC) was activated with limiting the initial 446 mA peak current passing through the varistor (MOV) to the peak 48 mA after 120 seconds and following reduction to the peak 10 mA (value of the varistor´s energy capability) - the varistor is still fully functional - after the overvoltage ends, no need for SPD replacement!

Oscillogram of sample SAFETEC®, current flowing through the SPD at applied 438 V


In the case of TOV resistance test, all four competitive surge protection devices have conducted and let through SPD high level of energy, depending of the source availability (danger!).

Current conduction caused destruction of the SPD, lead to the disconnection of thermal protection (sample No. 4), or fire and explosion (samples No. 1, 2, 3) - result: SPD replacement and danger!

But nothing above will happen if SAFETEC® solution is applied! 


VN:F [1.9.17_1161]
Rating: 0.0/6 (0 votes cast)

This post was written by:

- who has written 791 posts on PowerGuru - Power Electronics Information Portal.

Contact the author

Leave a Response

You must be logged in to post a comment.