Earthing

 

 

Q:        How do I eliminate unwanted static charges?

A:         Static charges can be eliminated from conductive bodies by just grounding it or bonding them to inherently grounded objects like water pipes, big metallic storage tanks which are placed directly on the ground, etc.... If the body has a positive charge it will achieve equilibrium by dragging electrons from the ground onto the body. Otherwise it will leak electrons from the body into the ground. For non-conductive bodies the static charges need to be eliminated by means of ionisers (they can produce positive and negative ions) or in the case of surfaces by painting them with a conductive metallic-film-compound and then grounding it. There is only one limitation to this grounding and that is no ESD grounding method can justify the endangering of personnel to mains electric shock.

 

Q:        What is soft grounding?

A:        Persons must be protected by an in-line resistance to ground as to limit the current passing through any parts of their body to not more than 3mA. The intention is to protect a person from electric shocks or other electrical related injures.

 

Q:        How do I prevent sparks occurrence?

A:        A good method of prevention other than grounding or neutralising charges on objects is to bond objects in between so they are at the same potential, also they may be bonded together and also be be grounded simultaneously as to make them to be near equal 0V potential. Sparks will not occur where the potential difference in between objects and/or earth is small enough. There are objects which are inherently grounded like water pipes, big metallic storage tanks placed directly on the ground that can be used for bonding. 

 

Q:        How corrosion may creep into earthing connections?

A:        The use of metallic materials for earthing inter-connections for ESD applications must be chosen carefully as bolts and washers  may corrode easily if wrongly used. For example, a conductive flooring which must be connected through a low resistance path to earth may be rendered inefficient if there is some aluminium connection sitting on concrete along the low resistance path, there may be bad corrosion. The necessary and continuous washing of the surface, even if soft chemicals are used, may also contribute to the low resistance path degradation as the earthing elements may also corrode.

 

Q:    How do I go about implementing an ESD protective ground?

A:    The ESD Association defines a Common Point Ground where all ESD protective elements must be connected to, this allows for all the elements interconnected through this point to experience a closely similar resistive ground path value, hence inhibiting the appearance of significant potentials in between the elements.  The ESD Association standards recommend bonding of the above mentioned common point to the local electrical ground (green/green-yellow wire), as all electrical appliances will be grounded to these local ground, all items should be closely at a very similar potential.

 

Q:    Is it a good idea to have installed a separate earthing rod in the ground just for ESD groundings? 

A:    No. If we provide a separate earth sink just for ESD grounding purposes there is the danger of appearing a large potential in between the local earth and the ESD second earth because of a difference in resistance in between the two, hence a large potential could develop in between a wrist strap or workstation surface mat and the electrical equipment on the same surface, a dangerous situation for the operator, undoubtedly. Of course bonding the two earths together will solve the potential difference or resistance in between problem.

 

Q:    Some people use insulated earthing wires/cables for ESD installations and some others use them bare. Which is the better?

A:    I would say that bare wires/cables would be better in these installations as they will allow for quick visual inspections when troubleshooting. Cable insulations have different levels of resistance to surrounding cable potentials but ESD discharges are mostly of unpredictable strength as they depend of many parameters, including the ambient.

 

Q:    How often should I audit the ESD ground system of my location?

A:    As checking out only one component of an ESD system will serve little purpose, a full audit must be effected, need to check out the whole ESD protection systems as well as components, the ground system is therefore covered. The advice of the ESD Association (ESDA) on document ANSI/ESD S20.20-2007 need to be followed closely. Audit may be applied via industry specialised personnel, external contractors or a mix of the two. Additionaly there are some daily inspections to some ESD functional aspects that are run by company internal ESD control management.  Typically a company will self assess a reasonable auditing cycle according to the size of the implementation and their personnel availability to do the job, also a crucial factor will be the sesnsitivity to static discharges of the equipment used or produced, eg. auditing once every 6-months is quite typical.

 

Q:    What is earthing bonding?

A:    Bonding is a term used to describe the technique of electrically interconnecting all objects on a given place to a same floating earth plane, this plane can be also connected to ground, the intention behind is to have all interconnected objects of this earthing plane at the same potential. There are a permanent (welding or brazing) and temporary bonding (bolts&spring washers, pressurised clamps and others which should have the ability of penetrating a rusty surface, lacker, paint or other insulating compounds so as to make a sound electrical contact).

 

Q:    What are the minimum distance restrictions, if any, to have in between the external ground plate electrodes of the general earth and the lightning protection arrester?

A:    There is considerable discussion on the ESD related forums over this issue. Standards at present do not seem to specify this. However, in my opinion, and based in ongoing discussions, they must be separated by a reasonable distance, where this should be at least 7m (20ft). However all other earths in the building should be bonded together as, for example, protect a wrist strap user. Should a lightning strike still leak into the general bonded earth system, users would have their potential raised as well, hence they would be protected.