Suppose someone operates isolator on load, it will produce ionize the surrounding air resulting generation of huge arc and heat which may result unplanned costly damage in the power system as there is no arc quenching medium in isolator. Heat produced will melt the contact of the isolator, may cause fire in that part of power system, damage to insulators or other faults.
Also huge amount of arc or heat produced means losses of electrical energy. Hence in-order to avoid these damages or faults due to produced huge arc and heat, an isolators should never open on load. That's why it is also known as no load device.
Conclusion
Hope, you have clearly understand why isolators are not opened on load. Make sure to leave your opinion and suggestion about this post in the comment section below.