Sizing Surge Protectors

[Murloc]

From my understand surge protectors are over voltage devices.

I have an site where an electrician has installed surge protectors that are rated for 600Y/376 V, 3 phase, 4 wire.
He claims surge protection devices can automatically sense voltages and disperse surge currents. Other people in the site
claim that they not longer have problems with lighting surges after they were in stalled.

The system is a high leg delta 240/120 V. 3 phase, 4 wire. From my understand those surge protectors shouldn't be doing anything
since it only senses a 600 voltage circuit.

Is there still benefits to these over sized surge protectors that I'm not seeing?

 

[petersonra]

From my understand surge protectors are over voltage devices.

I have an site where an electrician has installed surge protectors that are rated for 600Y/376 V, 3 phase, 4 wire.
He claims surge protection devices can automatically sense voltages and disperse surge currents. Other people in the site
claim that they not longer have problems with lighting surges after they were in stalled.

The system is a high leg delta 240/120 V. 3 phase, 4 wire. From my understand those surge protectors shouldn't be doing anything
since it only senses a 600 voltage circuit.

Is there still benefits to these over sized surge protectors that I'm not seeing?

It will still work, but it will just clamp at a higher voltage.

I don't recall off hand what the clamping voltages are but let's just make something up for illustration purposes.

A 208/120 type 1 SPD might have a clamping voltage of 500 V L-L. A 600/337 type 1 SPD it might be 1000 V L-L. So you could put the 600 Volt unit on a 208 Volt system but the device would not work until it saw 1000 V L-L instead of 500 V L-L.

 

[gar]

220919-1649 EDT

"Merlon:" I enter your name and this stupid software changes it. Tried several times and could not make my correction stick.

No one has responded to your question so I will try.

An MOV transient limiter is made from a material that has a VI curve that is a high resistance for voltages below a certain level ( in the manufacture of the MOV this voltage can be designed to be some specific value ), and then a much lower resistance for voltages above that level. An equivalent circuit for this can be approximated by a series circuit of a battery of the threshold voltage, a diode, and a series resistance. The larger the physical bulk of the MOV the lower is this series resistance.

The equivalent circuit I described is only single sided. Thus, for a more correct equivalent circuit you need a second circuit in parallel with the first circuit, but with the diode and battery directions reversed.

An actual MOV has a somewhat soft transition from high impedance to low.

To be an effective protective device you want this threshold voltage to be just somewhat above the peak of your nomlnal supply voltage. For a normal 120 V circuit the peak is about 170 V. If you assume that a 120 V supply might run as high as 132 V, then peak for a sine wave wave of 132 V would be about 187 V. Thus, it is quite common to use a 200 V MOV on a 120 V circuit. A surge limiter at 600 V is too high for your application.

Further you should use different voltage ratings for your suppressors in your circuit. For the 120 V legs use 200V units, and for the 240 circuits use 400 V units.

.

 

[Jraef]

… He claims surge protection devices can automatically sense voltages and disperse surge currents.”

To further this thought from your original post, he seems to imply that SPDs somehow measure the voltage and “decide” when to clamp. Nothing like that is taking place. As mentioned, MOVs are passive devices designed around specific properties to act at set levels of voltage. So using a higher voltage device is not recommended because it is intended to protect at a peak expected from those higher voltages. By the time they act, lower voltage devices might be damaged.