THESUBJECT G R O U N D I N G AND THE N E C
THE SPEAKER • BOB LUDECKE • Member IAEI • CA Certified Electrician • Lic. Electrical Contractor • CSLB Industry Expert • CSLB Subject Matter Expert • Hands on Electrician Since 1974
SPECIAL THANKS TO Thomas E. Trainor Chief Electrical Inspector City of San Diego, California Representing IAEI on CMP 7 Member - NFPA Member - U.L. Electrical Council
ALSO GUILTY OF: Member of the original NFPA Task Group on the Useability of the National Electrical Code Chaired the sub-committee assigned to review Article 250 and make recommendations to improve it’s “useability”
The ‘99 NEC Completely Reorganized Exceptions Minimized And Most Importantly....... Performance Requirements Added
Prescriptive vs Performance NEC rules are typically Prescriptive A Prescriptive Rule tells you what to do But, what it doesn’t tell you is..... Why you’re doing it!
Prescriptive vs Performance A Performance Rule is descriptive It describes what has to be accomplished And this description will most generally Explain the intent of the rule
Why is this Important? Grounding continues to be a Mystery Improper Grounding is Commonplace Proper Grounding is vital for an installation To protect from Fire and Electrocution
Why is this Important? It is my considered opinion, That a better understanding of The intent of the Grounding Rules Will lead to better, safer installations
TYPICAL CIRCUIT OPERATION • Only four things can happen when a circuit is energized. • It can operate normally • There can be an overload • There can be a short circuit • There can be a ground fault
HOW DOES GROUNDING FIT IN? • As long as the circuit is operating normally, • GROUNDING IS NOT NEEDED • As long as the circuit is operating normally, • GROUNDING IS NOT NEEDED
THE “UNGROUNDED” CIRCUIT T A circuit consisting of a transformer, 2 - 15A conductors and a light bulb will operate just fine (Check out the barn) Grounding is not needed To make it work or To make it safe
HOW DOES GROUNDING FIT IN? • Under an overload condition, • GROUNDING IS NOT NEEDED • PROTECTION FROM OVERLOAD IS PROVIDED BY • THE OVERCURRENT DEVICE • Note that current is only flowing on the conductors that we installed to carry current
HOW DOES GROUNDING FIT IN? • Under a short circuit condition, • GROUNDING IS NOT NEEDED • PROTECTION AGAINST SHORT CIRCUIT IS PROVIDED BY • THE OVERCURRENT DEVICE • Again, current is only flowing on the conductors we installed to carry current
HOW DOES GROUNDING FIT IN? • Under a ground fault condition, • GROUNDING IS NOT NEEDED • PROTECTION AGAINST GROUND FAULT IS PROVIDED BY • THE OVERCURRENT DEVICE • HOWEVER……...
RETURN PATH REQUIRED • THE OVERCURRENT DEVICE CAN ONLY PROTECT AGAINST A GROUND FAULT IF, • THE CIRCUIT IS INSTALLED SO THAT ALL METAL PARTS ARE BONDED TOGETHER AND TO THE SERVICE NEUTRAL, • WHICH CREATES A LOW RESISTANCE PATH FOR FAULT CURRENT TO RETURN TO THE SOURCE OF SUPPLY
LETS LOOK AT A TYPICAL CIRCUIT L O A D 100’ of Overhead Distribution Line, 25’ of Service Drop, 25’ of Service Entrance Conductor, 100’ of Branch Circuit Conductors
LETS LOOK AT A TYPICAL CIRCUIT L O A D Current flows…...
LETS LOOK AT A TYPICAL CIRCUIT L O A D From the transformer to our Service
PATH OF CURRENT FLOW - NORMAL OPERATION L O A D Through the Overcurrent Device to our Load
PATH OF CURRENT FLOW - NORMAL OPERATION L O A D Through the Load returning to the Service
PATH OF CURRENT FLOW - NORMAL OPERATION L O A D And back to the transformer
PATH OF CURRENT FLOW - NORMAL OPERATION L O A D What determines the amount of current that will flow in this circuit?
PATH OF CURRENT FLOW - NORMAL OPERATION L O A D The Total RESISTANCE or IMPEDANCE in the circuit will determine the amount of current that will flow in the circuit
THINGS YOU CAN COUNT ON • OHMS LAW WORKS • We can change the code, or • Hire a different contractor, or • Use romex instead of EMT, but • E = I x R still works
OVERLOAD AND SHORT CIRCUIT CONDITIONS L O A D How is our circuit protected against overload and short circuit?
OVERLOAD AND SHORT CIRCUIT CONDITIONS 15A Circuit Breaker L O A D THE OVERCURRENT DEVICE PROTECTS THIS CIRCUIT FROM BOTH OVERLOAD AND SHORT CIRCUIT
SUMMARIZING TO THIS POINT CIRCUIT CONDITION PROTECTION PROVIDED BY: GROUNDING? O/C PROT? NORMAL OPERATION NO NO OVERLOAD CONDITION NO YES SHORT CIRCUIT CONDITION YES NO
GROUND FAULT CONDITION So lets talk about a Ground Fault Condition Which certainly sounds like the one condition where Grounding would be important and decide for ourselves whether Grounding Provides Protection for Equipment or Personnel under a Ground Fault Condition
GROUND FAULT CONDITION L O A D What happens if the hot conductor comes into contact with our metal box?
GROUND FAULT CONDITION L O A D And our friend comes along and touches it? IS HE IN JEOPARDY?
GROUND FAULT CONDITION L O A D NO NOT AT ALL AND WHY NOT?
GROUND FAULT CONDITION L O A D Because the transformer we’re looking at IS NOTGROUNDED so there is NO PATH THROUGH EARTH for current to return to the transformer
GROUND FAULT CONDITION L O A D Yes, that was a “Trick” question Sorry about that But the intent was to make a point
THINGS YOU CAN COUNT ON • NO CIRCUIT - NO CURRENT • CURRENT DOES NOT FLOW UNLESS THERE IS A CONTINOUS PATH FROM ONE SIDE OF THE SOURCE OF SUPPLY TO THE OTHER • CURRENT CANNOT TRAVEL THROUGH THE EARTH TO RETURN TO A TRANSFORMER UNLESS THE TRANSFORMER IS GROUNDED
GROUND FAULT CONDITION L O A D So our friend in this situation is perfectly safe HOWEVER.....
GROUND FAULT CONDITION L O A D What do we know about utility company transformers?
GROUND FAULT CONDITION L O A D THEY’REGROUNDED And, with this transformer grounded, our friend is in serious jeopardy
SO WHY ARE THEY GROUNDED? • To minimize the damage caused if lightning strikes their distribution lines, or • If a 12 KV line drops onto a low voltage line, • In addition, grounding the neutral of the distribution system stabilizes the voltage. • So, basically for the same reason we ground services at buildings.
GROUND FAULT CONDITION L O A D Because utility transformers are grounded, we need to do something to our equipment to keep our friend from being electrocuted
GROUND FAULT CONDITION L O A D Can we protect our friend by grounding our metal equipment? Lets take a look.
GROUND FAULT CONDITION L O A D Grounding our equipment provides asecond path for fault current
GROUND FAULT CONDITION L O A D The first is through our friend to earth and back to the transformer
GROUND FAULT CONDITION L O A D The new second path is through our metal equipment to earth and back to the transformer
FAULT CURRENT PATH We need to open a 15A Circuit Breaker as quickly as possible. This will require a fault current of 60A to 75A. (4 to 5 times the rating of the breaker) We can use Ohm’s Law to find out how much current will flow on our new path.
GROUND FAULT CONDITION L O A D The voltage is 120V. We need to know the resistance in this circuit to calculate current
FAULT CURRENT PATH Assuming a minimum of 5 ohms resistance through each grounding electrode, we know there is at least 10 ohms resistance in the fault path that we created by grounding our equipment.
FAULT CURRENT PATH THEREFORE, USING OHM’S LAW: E = I x R and Transposing, I = E / R I (current) = E(voltage) / R(resistance) and so, I = 120 / 10 = 12A
FAULT CURRENT PATH ONLY 12 AMPS WILL 12 AMPS TRIP OUR 15A CIRCUIT BREAKER? ABSOLUTELY NOT
WITH EQUIPMENT GROUNDED L O A D So the Overcurrent Device does not open And we have fried our friend