# How does the surge protector on disconnect of my air conditioner condenser protect?

sevensages
TL;DR Summary
There is a straight path for electricity to travel from my breaker box to my disconnect to my condenser without going through a surge protector. How does the surge protector on my disconnect of my condenser protect my condenser?

I recently installed a surge protector to the disconnect for the condenser of my straight air-conditioner. I installed the surge protector to the disconnect for my condenser in exactly the way that the instructions with the surge protector instructed me to install the surge protector. I don't understand how the surge protector can protect my condenser from a surge.

The photograph above this text is a drawing or diagram I made of the electrical set up of my disconnect and my surge protector. As the drawing shows, there are four bolts holding wires in my disconnect. I made little circles represent the bolts holding the wires in my disconnect. The first bolt is the circle on the far left. The second bolt is the circle that is second from the left. The third bolt is the circle that is third from the left. The fourth bolt is the circle to the far right. When the disconnect is plugged in, a metal bar (which is a conductor of electricity) connects the first bolt to the second bolt, and another metal bar (which is also a conductor) connects the third bolt to the fourth bolt. The first wire of the surge protector goes to the second bolt. The second bolt merely holds the wire of the surge protector to the wire of the first leg of power going to the condenser. The third bolt merely holds the wire of the surge protector to the wire of the second leg of power going to the condenser.

I can think of two ways that a surge of electricity could go through my breaker box to the disconnect of my condenser: 1# a surge of electricity due to a lightning strike and 2# a surge of electricity due to a microwave or other electronic device or an appliance (such as the refrigerator) turning on.

If the surge protector was wired up so that current had to go through the surge protector to get to my condenser, I could understand how the surge protector would protect my condenser from a surge of electricity. If a surge of electricity went through my breaker box to my disconnect, I don't see any reason why the current would not bypass the surge protector and go straight to my condenser.

Since a surge of electricity from the breaker box would have a straight path from the breaker box to my disconnect to the condenser (without ever going through the surge protector), how would my surge protector on the disconnect for my condenser protect my condenser from a surge of electricity?

Mentor
It looks like it is connected correctly; it is in parallel with the AC Mains feed and the Earth Ground connection. Presumably there are 3 MOVs or other transient clamp devices inside the unit, one differentially between H and N and the other two between H-Gnd and N-Gnd. That offers protection from differential and common-mode Surge transients.

And it is correctly placed *after* the breaker box, so that if the Surge is big enough and the protection clamps sink currents over the rating of the breakers in that panel, the breakers will open.

Do you have the datasheet for the Surge protection module? What are its voltage, current and power-energy ratings?

Last edited:
sevensages, DaveE and Lnewqban
Staff Emeritus
There are different kinds of surge protectors. One of them is called a MOV, Metal Oxide Variable Resitor (or Varistor). It is simple an inexpensive.

From:
https://www.electronics-tutorials.ws/resistor/varistor.html

As such varistors are used in sensitive electronic circuits to ensure that if the voltage does suddenly exceeds a predetermined value, the varistor will effectively become a short circuit to protect the circuit that it shunts from excessive voltage as they are able to withstand peak currents of hundreds of amperes.

A disadvantage is that a surge can be so severe, that it burns the MOV out. If that happens, it no longer protects and must be replaced. But there is no indication telling you that it must be replaced. If you don't know, it can sit there year after year, not protecting anything.

Lnewqban, berkeman and phinds
Mentor
A disadvantage is that a surge can be so severe, that it burns the MOV out. If that happens, it no longer protects and must be replaced. But there is no indication telling you that it must be replaced. If you don't know, it can sit there year after year, not protecting anything.
Funny story -- We use MOVs a lot to protect our powerline communication modules and devices, so we have spent a lot of time testing the quality of the MOVs from different manufacturers. It turns out that there is a large variation in the quality of the MOVs, meaning one manufacturer's MOVs may have the capability to absorb many times more Surge hits than the others (I won't name any names, but one such excellent manufacturer rhymes with hooratta...).

So we spent several months a few years back running semi-automated Surge tests in our HW Lab, where the Surge tester would be set up to apply one hit per minute of varying amplitudes until the MOV failed. An engineering technician was in charge of running the tests, and would often start up a test and leave the Surge generator running on its own for 30 minutes before coming back to check it. We always cover the unit under test (UUT) with a heavy clear Lexan shield box, to prevent flying flaming bits of stuff from landing on nearby benches.

One problem is that the Surge generator is fairly quiet, with only a soft beep warning before each hit. So it's easy to be working in the lab and forget that the test is running (unattended). The other problem is that MOVs often fail short on the next-to-last hit, and then fail open with a spectacularly loud *BANG* on the last hit. So it was not uncommon during that test period for some poor engineer (me included) to be totally absorbed in the testing that they were doing in the lab, and all of a sudden the gunshot sound of a MOV blowing up would scare you right out of your chair into a crouch on the floor.

Many of us were relieved when that testing was completed!

Lnewqban
sevensages
It looks like it is connected correctly; it is in parallel with the AC Mains feed and the Earth Ground connection. Presumably there are 3 MOVs or other transient clamp devices inside the unit, one differentially between H and N and the other two between H-Gnd and N-Gnd. That offers protection from differential and common-mode Surge transients.

And it is correctly placed *after* the breaker box, so that if the Surge is big enough and the protection clamps sink currents over the rating of the breakers in that panel, the breakers will open.

Do you have the datasheet for the Surge protection module? What are its voltage, current and power ratings?
I don't have the datasheet right now. I am at work, so I cannot look at the surge protector right now. I think it might have the voltage, current, and power ratings on the outside of the surge protector.

What are H and N?

I am assuming that H-Gnd stand for H-ground, and N-Gnd stands for N-ground. But I don't know what H and N means.

Mentor
What are H and N?
Hot and Neutral (sometimes called Line and Neutral). Presumably that's what you have drawn for your single-phase power that is routing from the breaker box to the disconnect...

sevensages
There are different kinds of surge protectors. One of them is called a MOV, Metal Oxide Variable Resitor (or Varistor). It is simple an inexpensive.

From:
https://www.electronics-tutorials.ws/resistor/varistor.html

View attachment 295045

View attachment 295044

A disadvantage is that a surge can be so severe, that it burns the MOV out. If that happens, it no longer protects and must be replaced. But there is no indication telling you that it must be replaced. If you don't know, it can sit there year after year, not protecting anything.

My surge protector has a green light that lights up when the surge protector is functional and still protecting the condenser. If my surge protector gets burned out so that the surge protector no longer protects, then the surge protector lights up with a red light.

Mentor
Interesting. When you get a chance, please post a link to the datasheet. It will be interesting to see what they have inside that let's them show a status like that. Presumably more than just simple MOVs.

sevensages
Hot and Neutral (sometimes called Line and Neutral). Presumably that's what you have drawn for your single-phase power that is routing from the breaker box to the disconnect...
My disconnect has two hot lines. My condenser is powered by TWO legs of power.

Mentor
My disconnect has two hot lines. My condenser is powered by TWO legs of power.
So you mean 240Vac in the US? L1-L2? That's fine; nothing changes in my previous comments except for H,N --> L1,L2.

sevensages
Interesting. When you get a chance, please post a link to the datasheet. It will be interesting to see what they have inside that let's them show a status like that. Presumably more than just simple MOVs.
I will post a link to the datasheet if I can find it.

I still don't understand how the surge protector works. I am not an electrical engineer or a physicist like many of you people.

There are two motors in my condenser that would need protecting: the condenser fan motor and the compressor. As I recall, my compressor is rated to draw 6 amps. My condenser fan is rated to draw approximately 1 amp. The only way that I can imagine a surge protector protecting my compressor and my condenser fan motor is if the surge protector draws a substantial amount of current away from the compressor and condenser fan. But i think that the compressor would not run properly if the surge protector caused the compressor to draw substantially less than 6 amps. So I don't think that the surge protector reduces the amp draw of the compressor or the condenser fan. So how does the surge protector protect my compressor and my condenser fan?

sevensages
So you mean 240Vac in the US? L1-L2? That's fine; nothing changes in my previous comments except for H,N --> L1,L2.
Approximately, that is what I mean. It might be 220 Vac, not 240 Vac. But 240 amps is in the general ballpark.

Edit: The above sentence is a typo. I meant 240 VOLTS is in the general ballpark, not 240 amps.

Mentor
Approximately, that is what I mean. It might be 220 Vac, not 240 Vac. But 240 amps is in the general ballpark.
Is this an industrial installation? Or are you saying that you have a 250A breaker panel in your home?

sevensages
So you mean 240Vac in the US? L1-L2? That's fine; nothing changes in my previous comments except for H,N --> L1,L2.
In America, most houses are made so that most electrical outlets in a house has just one leg of power. But the really big appliances (which don't plug into wall electrical outlets) such as condensers, wash machines, air handlers, etc have two legs of power.

sevensages
Is this an industrial installation? Or are you saying that you have a 250A breaker panel in your home?
My air-conditioner at my house runs off two legs of power at approximately 220-240 volts. That is standard in America.

I don't know how many amps my breaker box is rated for.

Mentor
In America, most houses are made so that most electrical outlets in a house has just one leg of power. But the really big appliances (which don't plug into wall electrical outlets) such as condensers, wash machines, air handlers, etc have two legs of power.
Of course, I'm aware of that. I'm just surprised at the current rating that you mention for your home. Even 50A panels and breakers are pretty big, so 250A breakers in a home seems kind of big to me...

Mentor
I don't know how many amps my breaker box is rated for.
Oh, you mentioned 240A in your post above, but perhaps that was a typo. No big deal.

sevensages
Oh, you mentioned 240A in your post above, but perhaps that was a typo. No big deal.
Oh----LOL I did make a typo!

I meant 240 volts, NOT 240 Amps.

berkeman
Mentor
Yeah, that would have been one BIG air conditioner!

So anyway, please link to the datasheet when you get a chance, but so far it looks like you wired it correctly (and according to the directions). Is lightning a big issue in your area?

sevensages
sevensages
Yeah, that would have been one BIG air conditioner!

So anyway, please link to the datasheet when you get a chance, but so far it looks like you wired it correctly (and according to the directions). Is lightning a big issue in your area?
Lightning is no more or less of an issue in my area than in the average area in America.

Staff Emeritus
I neglected to say that there is a second kind of surge -- the current surge when turning on electrical devices, including the motor in a HVAC compressor. Here's a circuit to provide both types of protection.

NTC stands for Negative Temperature Coefficient, thermistors.

Note that even that two way protector still has two wires in and two wires out.

All circuits can be made more elaborate to include a function like an indicator light. The trick it to to that in a way that does not compromise the basic protection. Just by looking at the plastic enclosure and the green LED, there is no certain way to say what is inside that surge protector, how effective it is or how dependable it is.

sevensages
Maybe you can make sense of this.

Homework Helper
Gold Member
My air-conditioner at my house runs off two legs of power at approximately 220-240 volts. That is standard in America.

I don't know how many amps my breaker box is rated for.

The nameplate of your condenser unit shows an MCA value of normal amps consumed by both fan and compressor when simultaneously on.

Then it shows a maximum over-current protection value, which should be the maximum size of the fuses in the disconnect (if any) and the dedicated breaker in the main electrical panel.
Some units also show a minimum value for that (a nominal number between max and MCA), to indicate that your breaker or fuses should not be rated for less than that.

In case the surge protector induces a short, the breaker in the panel trips and power supply to the condenser unit is interrupted to protect the compressor.

berkeman
Gold Member
To try to answer your original question, a surge suppressor acts on an electrical circuit somewhat like a pressure relief valve acts on a pressurized water or air system.

That is it does nothing under normal conditions and when the voltage or pressure gets too high, it directs the over voltage or over pressure somewhere else.

Here is a short 6 minute video about how pressure relief valves operate.

The electrical surge protector you are using of course doesn't have any moving parts, instead it is a special material, usually a powder compressed into a disc, think making a ball of wet sand, or a snowball.

The individual grains have a natural and very thin insulating layer on them but the inside of each particle can conduct electricity. When the voltage gets high enough, it can jump right thru that thin insulating layer.

The name of this device is an MOV, or Metal Oxide Varistor.

When the two wires on the device are each connected to a power lead and the voltage gets high enough, the MOV conducts very well and can pass hundreds, up to thousands, of AMPS. (yours is rated at 10kA or 10,000A.) This short-circuits the voltage spike and limits the voltage to whatever voltage the MOV was designed for. (yours is rated 700V from either Line to ground and 1200V across the two Lines.)

Many voltage spikes are very short duration and just cause a small life reduction of the MOV. (Yes, MOVs do have a life limit that depends on the total amount of energy they dissipate and their design.)

Longer voltage spikes will trip the circuit breaker or fuse supplying the circuit because of the high current flowing thru the MOV.

The surge suppressor you have has three MOV's; one across the two power lines (L1 to L2), and one from each power line to Ground.

Hope this helps!
Tom

Lnewqban