Final answer:
Maintenance work on portable fire extinguishers is performed on an as needed basis and should be done by trained professionals.
Step-by-step explanation:
Maintenance work done on portable fire extinguishers is required to be performed on an as needed basis. This means that the maintenance should be done when necessary, such as when the extinguisher needs to be refilled, recharged, or repaired.
It is important to regularly inspect and maintain fire extinguishers to ensure they are in proper working condition in case of an emergency.
Contracting out the maintenance work to a certified service company is also a common practice to ensure that the work is done by trained professionals who have the knowledge and skills to properly maintain the extinguishers.