UL Clamping Voltage Ratings on Surge Protectors
Having been in the SPD industry I did a lot of product and MOV testing. The reason for the high values is that these devices are tested at either 10,000 amps for 5,000 hits of 16 to 20 uS duration, or 8 by 20 as we call it. 8 uS rise time and a duration of 16 to 20 uS at the 50% of peak voltage point. Another option is 15 hits at 20,000 amps, spread out over an hour.
The results from vendors is kept secret, as there is tough competition between SquareD, Seimens, APC and APT (which I worked for). Note that it is surge current of tight accuracy and specific waveforms used for repetitious testing, while a single voltage clamp test (At 1.00mA) was used to validate the survival of the surge protection device, and was done before surge testing to establish a baseline clamp value. After testing a drift in the baseline of more than 10% meant the device failed testing.
The severe testing is to prove to UL 1449 3rd and 4th edition standards that the device can withstand severe current (simulated lightning strike) by simply absorbing it or failing by blowing an internal fuse. Ferraz/Shawmut makes MOV's with thermal cut-offs built in so along with 30 amp 600 volt fuses per MOV bank it is supposed to shut down with no out gassing or explosions.
An MOV's 1 mA test is the rating on the MOV plus about 50%. That is called the "soft clamp" rating. Also, 150 volt MOV's are used on 120 VAC circuits. MOV's marked 320 are for 208 to 240 VAC. 420 volts MOV's are for 277 VAC (USA) and 347 VAC Canadian power feeds. We put 2 in series to handle 600 VAC delta power used in Canada or 480 VAC delta used in US industry.
You are correct in that the "hard clamp" voltage is way above the supply voltage. Even the "soft-clamp" voltage is 30% to 50% above the line voltage.
This is because decades ago the surge industry learned a hard lesson. MOV's rated at 130 VAC were bursting into flames because of slight rises and variations in a 120 VAC source. In my house it is as high as 125 VAC sometimes. Then UL stepped in and mandated the 30% to 50% safety margin. 130 VAC MOV's are no longer made or allowed to be used in any surge suppression device.
During the last 2 decades UL and now ISO have become very stern about testing, pass ok modes and allowed fail modes. If a huge surge suppressor is installed in a hospital at the service entrance panel it can only fail by shutting down quietly, no matter how severe the lightning strike may have been. These devices include mica barriers between phases to prevent flash-over and give the platinum fuses time to blow.
The end results of all of this regulation is a surge suppressor that may clamp higher than ideal (though expensive Sidac based units have a tighter clamp voltage), but in the long run safety dominates, especially in other countries where the power grid is a bit unstable.
Better safe than sorry. Never buy a surge suppressor that does not have UL and ISO stickers on it. The ISO labels are holographic so they cannot be copied and used on cheap and dangerous rip-offs.