packagessafety
All the time I hear that the tab on a TO-220 like package is electrically connected to the centre pin, for example "out" on voltage regulators.
Does mounting them to a heatsink (of which may come in contact with tracks on the PCB) or casing for heat distribution not matter? My worst case thoughts are more than one regulator mounted on the side of a casing, and 5V and 3V coming together (harmless, but something)
Warning
Equipment chassis is usually grounded to wiring ground via earth lead on power plug where provided.
In any sane regulatory environment, appliances without an earth lead are required by law to be "double insulated" so that internal metal parts are not touch accessible during operation. (Note: Double insulation nowadays seldom involves two layers of insulation - it is more a state of mind in manufacture and testing that leads to touch safe equipment. Usually :-). )
While a chassis should be "touch safe", never assume it is, because:
There are non-sane regulatory environments,
or ones where the authorities enforce the rules so little that they may be ignored and
It is excessively common for people to do silly and dangerous things with mains wiring, despite being aware that people are reported to have died due to such things.
They say:
Class I
Class II - See also: double switching (and double insulated)
A Class II or double insulated electrical appliance is one which has been designed in such a way that it does not require a safety connection to electrical earth (US: ground).
The basic requirement is that no single failure can result in dangerous voltage becoming exposed so that it might cause an electric shock and that this is achieved without relying on an earthed metal casing. This is usually achieved at least in part by having two layers of insulating material surrounding live parts or by using reinforced insulation.
In Europe, a double insulated appliance must be labelled Class II, double insulated, or bear the double insulation symbol (a square inside another square)..
Class III
A Class III appliance is designed to be supplied from a separated/safety extra-low voltage (SELV) power source. The voltage from a SELV supply is low enough that under normal conditions a person can safely come into contact with it without risk of electrical shock.
The extra safety features built into Class I and Class II appliances are therefore not required. For medical devices compliance with Class III is not considered sufficient protection.
In eagle 7 your problem is solved. klick on the symbol in the connect-dialog (red cyrcle), so you can change if the pins of the same signal have to be connected in the layout.
Best Answer
If your heatsink is connected to another net than the TO-220's tab you need some insulating materials to mount the TO-220 on the heatsink. In the old days they used a thin pad of mica for this, but today different materials (plastics) may be available that insulate and at the same time allow heat transfer to the heatsink. Martin mentions Kapton, but I'm not sure if that is a good heat conductor. Use thermal paste on both sides of the pad. You also need a round insulator to insulate the screw from the TO-220 as well. Some heatsinks come with the required stuff, or sometimes you can buy it in a small kit.