I saw a series of diagrams on a commercial / ham website explaining to hams how to properly power its products.
Above, an example of their explanatory figures for discussion. The red cross is mine, lest anyone think it is less than confusing and dangerous.
Power distributions systems vary around the world, and particularly in the system of earthing used.
The schematic given above has two earthing symbols used, the one on the left is usually used to mean a connected to equipment chassis or exposed metal, the one on the right is usually used to mean a protective earth connection to the greater mass of real ground.
Yet the annotation on the left hand chassis earth symbol is “Earth GND (inside wall socket)”, and it is shown between the appliance plug and its internal transformer. It is not entirely clear, but on two of three counts, it is appliance chassis ground, and if you did not have English language, the score would be two out of two for chassis ground.
Now what about the right hand earthing symbol, by convention it means a connection to the protective earth, but it shows that connection independently of the appliance plug.
Sure, lots of countries use a two pin plug, and in many if not most, one of those terminals is a neutral terminal which is grounded at the distribution transformer which may be adjacent to the premises or perhaps 500m or more down the street. The dependence on a connection to the neutral conductor within the appliance for grounding of exposed metal is fraught with problems. It DEMANDS a polarised plug set, but they are commonly not used, tampered with, or bypassed with adapters so that there is a real risk that exposed metal of one appliance may be connected to the neutral and another may be connected to active (or line) if its plug is reversed.
In some countries, it is permitted that the protective earth connection of an appliance can be made separately from the appliance plug by a wire and screw terminal but these fail to ensure that the appliance is properly earthed by virtue of the plug being inserted in the outlet alone.
The author of the article seemed to be writing from a Philippines experience, but don’t think that this situation is something of third world countries alone, look critically at your own power system and ask:
is exposed metal bonded to the neutral of a non-polarised two wire power plug?
The argument that this is made safe by the use of core balance protection on the outlet (RCD and GFCI are terms used in some countries) is a nonsense, the appliance depends on the outlet for its safety and that is lost if it is plugged into an unprotected outlet.
In Australia, two pin and three pin plugs are used with ordinary 230V general purpose outlets (GPO), polarised in both cases. Exposed metal MUST be connected to the earth pin in Class I appliances, it is not necessary in Class II (so called double insulated) appliances. Exposed metal must NEVER be connected to neutral or active. The earth pin of a GPO MUST have a low resistance connection to the protective earth system. (Most of Australia uses the MEN earthing system where the protective earth is bonded to the supply neutral at the main switchboard, and possibly at distribution boards in outbuildings with their own earth electrode. Additional RCDs have been mandated on ordinary GPO final sub circuits for very many years, though older installations may not have them… be aware.)
Keep in mind that ‘double insulated’ appliances have a weakness if exposed metal is not connected to the protective earth. Consider you are working on an aluminium ladder standing on a concrete floor, drilling a hold in the wall and you drill into a power cable. The drill bit may be in contact with a live conductor, the spindle and possibly metal cased gearbox may be live, the aluminium ladder on a conductive concrete floor (concrete is regarded as conductive in this context) and the operator is between the two possibly holding the metal gearbox case to steady the drill. Yes, an RCD on the cable damaged will probably protect the operator… but in Australia, RCDs are not mandated for circuits like an electric stove. Be aware. An RCD on the drill circuit is of little use in this specific scenario.
To the diagram above, it is dangerous misinformation IMHO, directed to an audience that might not understand the subject too well at all.
- If your AC power distribution system uses only two wires, here are some tip on things to watch out for.