if the chassis of a DC power supply is earth grounded , and lets say I am messing around with my power supply and I touch the positive voltage perhaps through a jumper wire or a component on a breadboard and at the same time i touch the chassis do i get shocked or will current flow throw me? I understand there is no path for the current to return to the source because i did not touch the negative side, so how does the earth ground help on the enclosure ? I am just having a hard time picturing what happens when the positive side of things touches the chassis ground and how it helps… i know what happens when positive and negative touch.
Electrical – Chassis grounding short circuit
chassisgroundpowerpower supply
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Warning
- FYI - some early AC/DC radio sets had live to mains chassis when operated from mains.
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
- These appliances must have their chassis connected to electrical earth (US: ground) by an earth conductor (coloured green/yellow in most countries, green in the U.S., Canada and Japan). A fault in the appliance which causes a live conductor to contact the casing will cause a current to flow in the earth conductor. This current should trip either an overcurrent device (fuse or circuit breaker (CB)) or a residual-current device (RCD) also named as residual current circuit breaker (RCCB), or (ground fault circuit interrupter (GFCI)) or also, residual current operated circuit-breaker with integral overcurrent protection (RCBO). which will cut off the supply of electricity to the appliance.
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.
Where is that quote from? It is self-contradictory, and pretty much just plain wrong.
Current will flow if a load is connected between a generic power supply positive and negative, no ground connection is needed, unless you have some special purpose power supply.
A ground connection is often used for safety, especially in line non-isolated supplies, or as a means to reduce noise. Even so, not all supplies pass the ground on to the output terminals or even make it available (for common examples, think wall-warts and such with just a two pole output). Many supplies don't even have an incoming ground terminal from the mains (small switchers, and again, wall-warts).
I was going to post this as a comment, but it seemed to cover the question.
Edit as per comment below and corrected text.
Well, that does change things. Generally, a DC power supply has no output tied to ground, or connected to the supply electrically at all, so connecting a load between an output and earth ground would result in no current flow. Supplies that have an output set of terminals and a separate ground can often be configured (by jumping ground to one of the output terminals) as positive ground or negative ground if desired.
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Best Answer
An earth grounded enclosure is there to protect you from an internal electrical short inside the enclosure.
If the enclosure has a proper Earth Ground connection to mains, then an internal short to a metallic chassis will be safely shunted away to earth ground. One should be able to still touch the metal case in such circumstances and not get killed.
The added benefit (and in low voltage DC applications, many times the main benefit), is EMI shielding; protecting outside circuits from the noisy radiating internals, and protecting your internal circuits from noisy radiating external sources.
As an aside, many power supplies allow you to connect either the DC+ or DC- output terminals to the chassis (earth) ground. This allows you to reference the output to chassis. In effect, biasing DC- output to the same voltage as the chassis, or alternatively, connecting the DC+ output terminal to chassis (producing a negative biased supply rail). This depends on the capability and design of your power supply. Most commonly, one would connect the DC- output terminal to chassis ground for a positive output supply. This is not exclusively for safety reasons, but for electrical reasons (learn about common-mode signals).
Good Luck!