On a point of order, the chair is a chair and he's also the questioner. Now, to do it properly, if the chair wants to question, he should move to the table and ask questions. If he's going to be a chair and a questioner at the same time, then in fairness he should let the witness answer, because there's nobody overseeing his interruption.
So I would ask the chair that if he wants to sit as a chair and a questioner, he should allow the witness to finish answering before he interrupts her, or he should take his place as a questioner and have somebody else oversee it. If you wish to continue as a chair, you should then use your discretion so as not to interrupt the witness and allow the witness to finish her answer.
Is that not fair?