On the other hand, the strength of PVC increases as the service temperature is reduced. Hence the ability to withstand internal pressure is greater at temperatures below 20°C. However, at the same time, the material becomes less tough and is more susceptible to impact damage.
It is not currently the practice in Australia to increase the operating pressure of PVC pressure pipes operating below 20°C. For example, a PN12 PVC pipe has a maximum allowable operating pressure of 1.2MPa at 20°C and at temperatures below this even though the strength is greater at lower temperatures.
The decrease in the impact resistance of PVC pipes at low temperatures is only a concern for those installations where impact might occur in service. (emphasis added)
I would think that any large and rapid change in pressure would qualify as an "Impact".