5A & 5B -- Grounding and ESD
Why might flat metal surfaces not bond as well as expected?
1. Contact Resistance -- touch only at the tips of peaks in the metal. 2. Suface Contaminants 3. Surface Hardess (two hard pieces, versus 1 hard 1 soft.) 4. Contact Pressure (not enough pressure)
4 Subsystems that compose the facility ground system are?
1. Earth Electrode 2. Lightning Protection 3. Fault Protection. 4. Signal Reference
What are the three primary functions of a ground system?
1. Personnel Safety 2. Equipment and Facility Protection. 3. Electrical Noise Reduction.
Why is lightning hazardous to electrical equipment?
1.5.1b, 2.1.1, 2.2.3 In the immediate vicinity of the point of entrance of the stroke current into the earth, hazardous voltage gradients can exist along the earth's surface. Ample evidence (2-1)* exists to show that such gradients are more than adequate to cause death
What affect can current have on a human?
Can cause injury or death.
What is grounding?
Grounding offers excess electricity the most effective and safest route from an appliance back to the ground by way of an electrical panel. Electrical grounding is a backup pathway that is generally only used if there is a fault in the wiring system.
What effect can ESD have on electrical components?
It can destroy components.
How can lightning damage be prevented?
Multiple connections to earth electrode subsystem, high peak power transfer capability, low impulse impedance to minimize magnitude of transient potentials.
How does a personal wrist strap help control ESD?
Provides a ground for personnel, to prevent ESD.
A facility ground system forms a direct path of (complete the sentence from 1.5.1)
The facility ground system forms a direct path of known low impedance between earth and the various power, communications, and other equipments that effectively extends in approximation of ground reference throughout the facility
Describe the 4 facility ground subsystems. (The first sentence after the topic in 1.5.1)
The facility ground system is composed of an earth electrode subsystem, lightning protection subsystem, fault protection subsystem, and signal reference subsystem.
What causes thunder?
Through some means, not clearly understood, these air masses generate regions of intense static charge. These charged regions develop electric field gradients of hundreds, or perhaps thousands, of millions of volts between them. When the electric field strength exceeds the breakdown dielectric of air volts/meter), a lightning flash occurs and the charged areas are neutralized.
What is contact resistance, what effect on bonds?
he term contact resistance refers to the contribution to the total resistance of a system which can be attributed to the contacting interfaces of electrical leads and connections as opposed to the intrinsic resistance. When two such surfaces are placed in contact, they touch only at the tips of the peaks - so called asperities. Thus the actual area of contact for current flow is much smaller than the apparent area of metallic contact.
What is bonding?
process of connecting conductive objects together. Process of connecting conductive objects together bonding refers to the process by which a low impedance path for the flow of an electric current is established between two metallic objects. Other types of bonding which involve simply the physical attachment of one substance or object to another through various mechanical or chemical means are not treated.
What is the purpose of a lightning protection subsystem?
provides a nondestructive path to ground for lightning energy contacting or induced in facility structures
When achieving building grounding-the lower the resistance, the _________.
the lower the peak voltage and consequently the less severe the surface gradients
Why would test benches need to be grounded?
without grounding a test bench, someone risks having a current run through to whatever appliance they are working on and fry its circuits. the user could also end up getting shocked by the current
