An insulating tube (18) located between the main contacts (12b, 14b) and the arcing contacts (12a, 14a) of an interrupting chamber (10) of a high-voltage or medium-voltage circuit-breaker makes it possible to modify the distribution of the equipotential lines (V) during breaking. Thus, it is possible to reduce the electric field on the contacts, and thus to improve the breaking and the dielectric strength in the open position. The insulating tube (18) can also serve for transmitting movement between the contacts (12, 14) for triggering the circuit-breaker.