Electronic – SMPS Testing: Differential Probes or Common lead and best Grounding practice

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.

Wikipedia - appliance classes

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.

Circuit breakers are not enough to protect life. Circuit breakers are there to stop the cable in the walls of your house melting and possibly catching fire – circuit breakers and fuses perform the function of stopping a fire (which of course is also very dangerous to life).

For direct contact with a live AC part, in the UK we have residual current devices (RCDs) – these "trip" the supply if the current taken down one of the AC wires is different to the current down the other by ~20mA:

_{(source: diyhowto.co.uk)}

Clearly a fuse wouldn't be useful because the normal current of the devices attached to the AC will be tens or more amps. So if you have an appliance taking 10 amps and you touched one of the AC conductors you'd draw an earth current of maybe 20mA and this would "imbalance" the RCD and trip the supply.

As for touching both terminals simultaneously a different scenario has to be envisaged. I'm talking about AC power systems where one conductor (sometimes called neutral) is "earthy" i.e. it may have a voltage of only a couple of volts to earth – if you touched only this wire then it is very unlikely to trip the RCD BUT who cares – it's only a couple of volts put across your body at best and hardly any current will flow. If instead you touched both AC wires (live and neutral) then there will be an earth current taken from the live that is still significantly greater than the earth current from the neutral and the RCD trips.

Having said all of this ~20mA is still going to sting even if it is only for sub 100 milliseconds. Will it be lethal – possibly to people with heart complaints but will those folk be rummaging under a desk to blindly push a connector into a socket?

For AC systems that are "isolated" from earth, touching any one wire will barely be noticeable, but touching both will not trip an RCD and you'll be in serious danger – the current flow will be directly through the body and from conductor to conductor. Luckily these sorts of installations are not very common but certainly not unheard of. Losing the neutral-earth bond can cause this problem.