There has been a change in the way TVSS's are being tested now by the addition of new specification required by UL 1449 3rd Edition. Measured limiting voltage testing is now preformed at 6kV/3kA. Also, this test is now an ANSI standard which has not been the case in the past.
There has also been a change in the terminology from Transient Voltage Surge Suppressors to Surge Protection Devices.
There has been the addition of Nominal Discharge Current to ratings and markings on the devices.
This testing now includes Duty Cycle testing at nominal discharge current.
UL 1449 3rd Edition also gives four designations to surge protective devices (SPD's) depending on where in the electrical system the device is connected.
Type 1 - Permanently connected device installed before of after the service disconnect overcurrent device and intended to be installed with no external overcurrent protective device. This type of SPD most closely relates to devices tha were called secondary surge arrestors prior to 3rd Edition.
Type 2 - Permanently connected device installed after the service disconnect overcurrent device. This type of SPD most closely relates to devices that were called transient voltage surge suppressors prior to 3rd Edition.
Type 3 - Point of use SPDs that are installed with a minimun of 30 feet of conductor length from the service panel. These 30 feet of conductor lenght does not include conductors used to attach the SPD. Some examples of Type 3 SPD's are cord connected, direct plug-in and receptacle type SPD's.
Type 4 - Component SPD's and component assemblies.
There is quite a bit of information about this that can be found on the web. If you want to know more about the testing, do a Google search and UL 1449 3rd Edition and several good articles can be found. EC&M has a great article on this and Eaton does as well.
The point of this is that all manufacturers must now meet this testing with the product they are producing at this time. They still can sell all the product they have produced prior to this going into effect on 9-29-2009. Some manufacturers were building in fuses to meet the 2nd Edition criteria which was a cheap way of getting around it because if the fuse blows your protection is lost and most times you don't know about it.
TPS has met the new standards and is still the leading manufacturer for the best and most reliable TVSS/SPD's on the market.