The NSG500B unit contains two generators in a compact form; one for the microsecond pulse range (pulses 1 and 2) and the other for the fast pulse bursts in the nanosecond range (pulses 3a and 3b), together with a common control section. The rise time, pulse width and amplitude of the pulses 1 and 2 can be varied within wide limits. Facilities are provided to connect external impedances to match the generator to the conditions such as are to be found in a vehicle's supply system. The pulse bursts, 3a and 3b, can be set in amplitude and polarity to suit the test specification. The front panel is equipped with a presettable counter which enables automatic tests to be carried out for a predetermined time or number of cycles. Triggering can be set for manual or external operation and a trigger output is provided to drive an oscilloscope. Dampening of the interference pulses by the supply source is prevented by a built-in network.