Employees whose work means that they have to continue working at the time of Jumu’ah

The basic principle is that Jumu’ah prayer is obligatory upon
individuals, because Allaah says (interpretation of the meaning): 

“O
you who believe (Muslims)! When the call is proclaimed for the Salaah
(prayer) on Friday (Jumu‘ah prayer), come to the remembrance of Allaah
[Jumu‘ah religious talk (Khutbah) and Salaah (prayer)] and leave off
business (and every other thing). That is better for you if you did but
know!”

[al-Jumu’ah 62:9]

And Imam Ahmad narrated
from Ibn Mas’ood (may Allaah be pleased with him) that the Prophet
(peace and blessings of Allaah be upon him) said concerning some people who
were failing to attend Jumu’ah prayers: “I had thought to tell a man to lead
the people in prayer, then I would burn down the houses of men who fail to
attend Jumu’ah prayers.” (Ahmad, 1/402; Muslim, 1/452). Muslim narrated from
Abu Hurayrah and Ibn ‘Umar (may Allaah be pleased with them) that they heard
the Messenger of Allaah (peace and blessings of Allaah be upon him) say
from the minbar: “There are people who should stop neglecting Jumu’ah, or
else Allaah will put a seal on their hearts, then they will surely be among
the negligent.” And there is consensus among the scholars on this point
(that Jumu’ah is obligatory). But if a person for whom Jumu’ah is obligatory
has a legitimate shar’i excuse – such as one who is directly responsible for
a matter of national security and protecting the interests of the ummah, and
he is required to do that job at the time of Jumu’ah, such as those who are
in charge of security, traffic, wireless and telephone communications, etc,
those whose shift happens to be at the time of the second call to Jumu’ah
prayers or the time of a prayer in congregation – then such people are
excused for not praying Jumu’ah or prayers in congregation, because of the
general meaning of the verse (interpretation of the meaning): 

“So keep your duty to
Allaah and fear Him as much as you can”

[al-Taghaabun 64:16]

And the Messenger of
Allaah (peace and blessings of Allaah be upon him) said: “Whatever I
forbid you to do, avoid it, and whatever I command you to do, do as much of
it as you can.” Moreover, such a person has no less an excuse than one who
is excused because he fears for his life or his wealth etc., who the
scholars have said is excused for not praying Jumu’ah or prayers in
congregation so long as the reason is present. But the obligation to pray
Zuhr still stands, and he must pray it on time; whenever he can pray in
congregation he is obliged to do that, as is the case with all the five
daily obligatory prayers.