TVSS Specification

[Kiribanda]

Gentlemen,

This question is about TVSS/ SPD (Transient Voltage Surge Suppressors/ Surge Protective Devices) mainly used in LV (600, 208 V 3-ph) industrial installations where sensitive electronic load centers (e.g. Work stations, Servers etc) are served.

Most of the TVSS units available in the market for such applications (e.g. LIEBERT, EFI etc) offer their products for very high surge currents like 100 kA, 160 kA, 180 kA.

My questions are,

1)Can we use these TVSS units on ungrounded systems?

2)How can we explain such very high surge currents on LV networks even with the ungrounded method?

3)Is there any special duty requiremnt for these TVSS units so that they can be used on ungrounded systems?

3)If such a TVSS unit is installed on a 120 V network, then how do we make sure that the Max. Clamping Voltage (Max. Discharge Voltage) will not induce a high voltage across the protected equipment for a 160 kA surge current?

4)One UPS spec requires a 6 kV/ 10 kA TVSS unit at its 208 V, 3-phase input. What does this 6 kV indicate?

Your valuable inputs are greatly appreciated.

Kiri

 

[RonShap]

From a Square D Publication:
SPD devices should not be applied to any ungrounded system.
The reason lies with the modes of protection for SPDs, which are line-to-line (L-L) and line-to-ground (L-G). A transformer with a secondary winding configured as a three-phase, three-wire ungrounded delta or wye has no reference bonded to ground. The transformer, however, is coupled to ground through the capacitive reactance of the distribution equipment. The capacitive reactance is not low enough to provide an adequate low impedance path around the L-G connected to metal oxide varistors (MOVs) for surge voltages that impinge on the primary side of the power class transformer. The SPD connected on the secondary side of an ungrounded transformer can be subjected to transient overvoltages in excess of those typically envisioned for low voltage distribution systems. If a ground fault were to occur on one of the secondary phase conductors of an ungrounded power system, the potential on the other phases can rise to a value between 80 percent and 173 percent above nominal phase-to-phase voltage. The charging of the capacitive reactance to ground through the power system inductive reactance can cause a potential rise with respect to ground as high as six to eight times the nominal phase-to-phase voltage in ungrounded systems and 12 to 16 times the nominal phase-to-phase voltage in ungrounded wye systems. Such parasitic resonant circuits can distort the normal line to ground voltage, causing it to rise to a destructive level. These overvoltage conditions can cause the MOVs of an SPD to conduct to the point of self-destruction and overstress the insulation of other electrical equipment.

 

[alehman]

Surge protectors used on ungrounded systems are available, but must be designed for 3-wire systems. Such high surge currents are rare, but can happen. They are normally very short in duration (microseconds). It is a concern for ungrounded system because the surge voltage can be between phases. I don't know what the 6kV means.

 

[RonShap]

The 6kV and 10kA is the UL method of Maximum Surge Current testing.

http://www.leviton.com/sections/prodinfo/surge/ul1449.htm

 

[LionelHutz]

You can use a TVSS on an ungrounded system if the TVSS is designed for the phase-phase voltage.

Lets use 3-phase 208V as the example.

In a grounded system, you would get 120V phase to ground and 208V phase to phase. So, you can install a surge protector phase to ground that clamps at some value above 120V.

In an ungrounded system, you are not limiting the phase to ground voltage to 120V. You could ground one phase and get a maximum continuous phase to ground voltage of 208V. So, the surge protection for this system needs to clamp above 208V.