1

Stop work and initiate emergency response when unplanned contact with a buried facility occurs. OR Stop work and initiate emergency response if unplanned contact with a buried facility occurs.

  • The grammar aside, the meaning of the sentence is quite strange. – Jason Bassford Dec 18 '18 at 7:00
1

According to definition 2 for "when" and 1a for "if" as listed below, the words may be used interchangeably.

It should be noted however that there is the phrase "when, not if" which is used to denote the inevitability of something, which may or may not be what you're looking for.

TL;DR

Both are valid, but using "when" might be interpreted as something that will happen, while "if" leaves the possibility of something not happening open.

Merriam-Webster's definitions for the conjunction "when" are

a : at or during the time that : while

went fishing when he was a boy

b : just at the moment that

stop writing when the bell rings

c : at any or every time that

when he listens to music, he falls asleep

2 : in the event that : if

a contestant is disqualified when he disobeys the rules

3a : considering that

why use water at all when you can drown in it— Stuart Chase

b : in spite of the fact that : although

quit politics when I might have had a great career in it

4 : the time or occasion at or in which

tomorrow is when we must decide humor is when you laugh

and those for the conjunction "if" are

1a : in the event that

b : allowing that

c : on the assumption that

d : on condition that

2 : whether

asked if the mail had come

I doubt if I'll pass the course

3 —used as a function word to introduce an exclamation expressing a wish

if it would only rain

4 : even though : although perhaps

an interesting if untenable argument

5 : and perhaps not even

few if any changes are expected

—often used with not

difficult if not impossible

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