Grounding, bonding, and ground fault currents

Grounding, bonding, and ground fault currents • Why all the confusion?

Why learn about Grounding? Grounding continues to be a Mystery Improper Grounding is Commonplace Proper Grounding is vital for a proper installation To protect from fire, shock, & even death

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

T THE “UNGROUNDED” CIRCUIT A circuit consisting of a transformer, two 14 AWG wires 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

SO, HOW DOES GROUNDING FIT IN? Under an overload situation • Note that current is only flowing on the conductors that we installed to carry the current Grounding is not needed Protection from the overload is provided by the Over Current Device (circuit breaker)

Under a short circuit situation Grounding is not needed Again, protection from the short circuit is provided by the Over Current Device Again, the current is only flowing on the conductors that we installed to carry the current

Under a ground fault situation Grounding is not needed Once again, protection from the ground fault is provided by the Over Current Device However

THE OVERCURRENT DEVICE CAN ONLY PROTECT AGAINST A GROUND FAULT IF, • THE CIRCUIT IS INSTALLED SO THAT ALL METAL PARTS ARE BONDED TOGETHER 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

L O A D Current flows…...

L O A D From the transformer to our Service

THIS WOULD BE THE PATH OF CURRENT FLOW UNDER NORMAL OPERATION L O A D Through the Over Current Device to our Load

L O A D Through the Load returning to the Service

L O A D And back to the transformer

L O A D What determines the amount of current that will flow in this circuit?

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 OVER CURRENT DEVICE PROTECTS THIS CIRCUIT FROM BOTH OVERLOAD AND SHORT CIRCUIT

SUMMARY PROTECTION PROVIDED BY: CIRCUIT CONDITION Grounding? Cir Breaker NO NO NORMAL OPERATION NO YES OVERLOAD CONDITION NO YES SHORT CIRCUIT CONDITION Let’s talk GROUND FAULT CONDITION

THE 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

No Circuit - No Current • CURRENT DOES NOT FLOW UNLESS THERE IS A CONTINUOUS 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

Why are Transformers 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 shocked or 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

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

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.

THEREFORE, USING OHM’S LAW: E = I x R and Transposing, I = E / R I (current) = E(voltage) / R(resistance) I = 120 / 10 = 12A

ONLY 12 AMPS ABSOLUTELY NOT WILL 12 AMPS TRIP OUR 15A CIRCUIT BREAKER?

WITH EQUIPMENT GROUNDED L O A D So the Overcurrent Device does not open And we have fried our friend

CONCLUSION GROUNDING DOES NOT PROTECT EQUIPMENT OR PERSONNEL FROM A GROUND FAULT

THE BONDING CONNECTION L O A D The vital connection left out of our discussion until now is the bonding of metal equipment to the service neutral

THE BONDING CONNECTION Every piece of conductive metal which is a part of our system or likely to become energized Must be connected together by an electrically continuous metal-to-metal contact or by an equipment grounding conductor

THE BONDING CONNECTION These connections create an electrically continuous, low resistance path from every part of our system back to the service equipment At the Service, these connections terminate on the Neutral Bus

THE BONDING CONNECTION L O A D These bonding connections let us use the neutral as a return path for fault current

THE BONDING CONNECTION L O A D Bonding provides a third path for fault current to return to the source of supply

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.

Grounding, bonding, and ground fault currents