Explosive charge

Explosive charge is a miscellaneous item. It is obtained in the Fallout 3 add-on, Operation: Anchorage, and in Fallout: New Vegas

Operation: Anchorage Characteristics
Explosive charge (listed ingame as "Explosive Charges") is a quest item used to destroy three artillery guns and two fuel tanks. Once placed, it appears as a bundle of dynamite on the object intended to be blown up. Once armed, it cannot be disarmed and will detonate in 20 seconds. It is armed as soon as it is placed; no further action is necessary to arm it.

Fallout: New Vegas Characteristics
Explosive charge is a quest item used to destroy the monorail in the quest I Put a Spell on You. It is located in the garbage can next to the terminal, and is placed in the vent inside the monorail.