Satellite Dish Clarification

[AndyD]

Sorry about the delay, I've not had time to make a decent reply to this topic.

Earthing

In simple terms, earthing is an integral part of every electrical supply circuit. It's the conductor that is never fused or switched and it connects the chassis of every item or appliance to earth potential (0v or ground). The idea is that if there's ever an internal fault that causes the live or current carrying parts of the supply circuit to come into contact with the chassis then the earth will allow a fault current to flow safely to ground and hopefully one that's large enough for the circuit protection to trip and disconnect the circuit.

The earth connection also prevents the appliance chassis developing a dangerous voltage under fault conditions with respect to ground. Anyone coming into contact with a faulty appliance is standing on the floor and can create a path through their body for current to flow which can cause injury if they touch it. The earth connection is designed to offer a lower resistance path for fault currents hence reducing the likelihood of injuries occurring.

Finally the earth is also functional. It's normal for small currents to flow down the earth wire even when there is no fault as such. Many electronic devices such computer power supplies and lighting ballasts etc have built in surge arrestors. These surge arrestors allow any high voltage surges or spikes to be connected and sink directly to the earth therefore preventing damage to the equipment.

So in short the earth is both a functional and protective part of an electrical circuit or installation which protects the user from coming into contact with dangerous voltages and causes rapid disconnection in the event of a fault.

Some appliances do not require an earth connection because they're constructed in such a way that it's highly unlikely they can develope a fault that could ever give the user a shock. Often these items are double insulated or have a plastic chassis or case which is non-conductive so even if an internal fault occurs it can't result in a hazard to the user. These appliances usually have a special identifying symbol consisting of two squares, one inside the other and are known as class 2 insulated. These appliances could have a two pin plug because an earth is considered unnecessary in this case and TV decoders and laptops etc often fall into this category.



Bonding

Bonding is simply connecting metallic objects together with a low resistance wire or conductive strap. This ensures they can't develop a voltage with respect to each other.

If objects are connected together and also a connection is made to earth then this is called earth bonding because all the objects connected together are also tied to zero volt earth or ground. Same applies to a single object that is connected to earth, this object is then held at earth potential therefore it's earth bonded. The earth bonded items can't assume any voltage other than zero with respect to earth whereas unearthed bonding will allow the items connected together to 'float' at any voltage applied to them although they will always remain a no voltage with respect to each other.

Bonding is often misunderstood, we don't bond small objects that are unlikely to suffer a rise in voltage or potential, for example bonding is not required on most building elements such as metal shelving or door / window handles. If it's a metallic object that leaves the premises such as a water or gas pipe then it would require bonding. This is often referred to as the difference between objects that are 'extraneous conductive' or 'exposed conductive' and ones that are 'non conductive' in the electrical industry.

The decision whether to bond or not is an important one and requires testing of the object in question and understanding of equipotential zones in order to decide which of the above categories an object falls into. Unnecessarily earth bonding can actually introduce a hazard of shock where there wasn't one previously. For example you won't get a shock if you are on an insulated floor surface whilst touching something live and you simultaneously touch a metal door handle on a wooden door but if the handle is earth bonded it them provides a low resistance earth path and a fault current will then flow and you'll get a shock. In this case if you add supplimentary bonding to the door handle you've turned a non-conductive part into a conductive part by connecting it to earth, this unnecessarily increases the risk of shock.

Bonding is a fairly complex subject on its own and in the interest of keeping this explanation simple and understandable there's some factors and aspects I've glossed over. It's critical it's done correctly for the safety of the buildings occupants and not something a person without the relevant competence such as handyman or DIY homeowner should attempt.