If a person has a cast on his hand, and the cast needs to stay dry, must he perform Netilat Yadayim on the other hand when he wakes up in the morning and before he eats bread? Does the Misva not apply if he is unable to wash both hands, or is he required to wash the other hand?
The Ben Ish Hai (Rav Yosef Haim of Baghdad, 1833-1909) addresses this question (in Parashat Tazria, 13) and writes explicitly that a person with a bandage on one hand must wash the other hand, both when waking in the morning and before eating bread. He adds that the person in this situation recites a Beracha over the washing, as usual.
Having established that the obligation of Netilat Yadayim applies even when one can wash only one hand, the question then arises as to whether the text of the Beracha should be changed in such a case. The standard text of the Beracha, of course, is "Al Netilat Yadayim," which mentions "hands" ("Yadayim"), in the plural form. When a person can wash only one hand, we might wonder if he should perhaps recite the singular term, "Yad," instead of the word "Yadayim," such that he recites the Beracha "Al Netilat Yad."
At first glance, we might compare this case to the situation of a person who immerses one utensil in a Mikveh. When one immerses several utensils at one time, he recites the Beracha "Al Tebilat Kelim," using the plural term, "Kelim" ("utensils"), whereas one who immerses just a lone utensil changes the text of the Beracha to "Al Tebilat Keli," using the singular term, "Keli" ("utensil"). This Halacha perhaps sets a precedent that is relevant also to Netilat Yadayim, such that a person who can wash only one hand should change the text of the Beracha to "Al Netilat Yad." We may, however, refute this argument by distinguishing between the two cases. While it is common for people to immerse a single utensil, it is rare that a person is able to wash only one hand. Quite possibly, then, the Sages from the outset formulated different texts of the Beracha over immersion – one for the immersing of a single utensil, and another for the immersion of multiple utensils – but formulated just one text for Netilat Yadayim. Since it is rare that a person cannot wash both hands, the Sages perhaps did not find it necessary to institute a separate text to be recited in this case.
A proof in the opposite direction might, at first glance, be drawn from the Halacha relevant to a person who is unable to wear the Tefillin Shel Yad, such as if his arm is in a cast. In such a case, he recites a Beracha over the Tefillin Shel Rosh – the Beracha of "Al Misvat Tefillin." Although he wears only one of the two Tefillin, he recites the plural term, "Tefillin," and not the singular term, "Tefila." This might indicate that the plural term is used even in the exceptional case when there is only one item, such as in the case of Netilat Yadayim when one hand is in a cast. However, this argument, too, may be refuted. The word "Tefila," of course, means "prayer," and thus it is possible that the Sages did not wish to institute the text "Al Misvat Tefila" because this would give the appearance of a Beracha recited over prayer.
Regardless, the Ben Ish Hai, in one of his responsa (Rab Pe’alim), writes (listen to audio recording for precise citation) that one who can wash only one of his two hands nevertheless recites the standard text of the Beracha, "Al Netilat Yadayim." This is the ruling also of the Kaf Ha’haim (Rav Yaakob Haim Sofer, Baghdad-Jerusalem, 1870-1939), in Siman 4, and of Hacham David Yosef, in Halacha Berura (vol. 1, Oserot Yosef, 8; listen to audio recording for precise citation).
Summary: If a person has a cast on one hand, and he thus cannot wash that hand, he performs Netilat Yadayim on the other hand, and recites the standard Beracha, "Al Netilat Yadayim." This applies both to the hand washing in the morning, and to the hand washing before eating.
