When performing ac/dc difference measurements between two standards, it is of importance to rapidly switch from dc to ac or vice versa, in order to avoid thermal cycling of the standards thus saving time and obtaining a better reproducibility between succesive measurements. This is specially advisable and convinient at high voltage (up to 1000 V).
A fast (∼ 1ms) ac/dc switching unit based on high voltage vacuum relays has been realised at OFMET (CH) which covers, with few modifications, the basic needs of other laboratories measuring ac/dc voltage differences. Taking into account suggestions of other laboratories, this switching unit could be fabricated at OFMET in a small series.
The number of units which can be manufactured at OFMET is restricted through limited manpower.