EMIT - Questions And Answers

If possible could u please explain the below sentence in a little more detail. "You will want to be careful of ground loops for possible electrical noise problems both to sensitive electronic equipment and ESD grounded operators." - Clayton Gardner, Australian Electronic Manufacturing Services, Sydney, Australia

This really depends on the Class of the devices you are handling. If you are handling Class 2 or higher devices (< 4,000 Volts), then this should not be an issue for you. Below is a scenario with a very sensitive Class 0 device. When working with Class 0 ESD sensitive devices, i.e., > 250 Volts (ESD STM5.1-1998), some of these devices could be sensitive to as little as 10 Volts. A ten Volt ESD (electrostatic discharge) can easily be set up from ground loops. If a ground point has more than one path to bus bar, you can create "small circuits" or loops within the grounding system. These "loops" can create potential differences from millivolts to several volts (beyond 10 Volts). If an ESD Sensitive device comes between to points in the grounding system where the potential difference is say 10 Volts, the device is at risk. In addition to ground loops, a grounding system can also act as a large antenna. If any of the ground connection points are ohmic (say because of a bad solder joint or a loose mechanical connection) you can create standing waves (depending on the length of the conductors) or other types of electrical noise which can be delivered through your ground into the devices being handled (if a direct connection is made). Common Point grounding as well as a good physical layout helps to reduce ground loops . The physical layout of the grounding system should have a main ground trunk with tributaries coming off in parallel to this main trunk to provide the common point grounds at your work stations. Hard grounds to these common point grounds should then be employed. Another reason for "hard" grounds (direct connections with essentialy no ohmic characteristics) versus "soft" grounds (direct connections with ohmic characteristics typically at 1 M Ohm) is to eliminate any potential problems (no pun intended) associated with an ohmic potential (voltage) difference in the ground connection. Usually a 1 M Ohm load is OK in a ground cord because of the typical discharge times and the associated with this RC electronic time constant. The 1 M Ohm load is a safety feature to the operator to reduce the current they may encounter when grounded and handling a live circuit (120 VAC).

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