Bilge pumps are used to remove bilge water (the bilge is the lowest compartment inside the hull of a boat, where the two sides meet at the bottom). Fuel and other contaminants can mix with the bilge water, therefore, electric bilge pumps are designed to be non-sparking. Bilge pumps for boats are often fitted with float switches which turn on the pump automatically when the bilge fills to a set level. Because bilge pumps can fail, a secondary marine bilge pump is recommended. The primary electrical bilge pump should be located at the lowest point of the bilge. The secondary pump should be located at a higher level so it runs only when the primary pump is overwhelmed. This keeps the backup bilge pump free of the debris in the bilge that may clog the primary bilge pump.