When entering a manhole, you should test for an explosive atmosphere:

Testing for an explosive atmosphere before removing the manhole cover is essential to ensure safety. This preemptive measure allows operators to assess the air quality in the vicinity and to identify any potentially hazardous conditions that could lead to an explosion or fire.

By conducting this test beforehand, you can determine whether it is safe to proceed with further actions, such as removing the cover or entering the manhole. If an explosive atmosphere is detected, appropriate safety measures can be taken to mitigate the risks, such as ventilating the space or using specialized equipment. This approach prioritizes personal safety and minimizes the likelihood of accidents.

In contrast, testing the atmosphere after the cover is removed could expose personnel to immediate danger, particularly if hazardous gases are present. The objective is to avoid any situation where a person may be put at risk due to an undetected explosive environment. Therefore, the correct answer underscores the importance of proactive safety measures in confined spaces, particularly in industrial settings.