`ELECTRICAL GROUNDS
ON
DRILLING RIGS
ELECTRICAL GROUND
GROUND – RELAY OPERATION
The 17LV68E2 ground relays are magnetically operated devices that operate when an electrical ground occurs. These relays have a lathching mechanism which must be mechanically reset after being tripped by an electrical ground.
The ground relay is connected to the common negative bus of the electric drilling motor and generator power circuit combination throught a disconnect switch. One end of the relay coil is connected permanently to ground.
The main purpose of this relay is to remove power an the event of a flashover (electricity short-cutting across the sureface of a commutator) in either a motor or generator. It also serves as insulation failure protection for the entire system. With the disconnect switch closed (normal operating position), a ground in a lead ,cable, or field winding will complete the circuit through the relay coil. This energizes the coil (set to operate at 250 milliamperes or less) and immediately opens the relay contacts and removes power. The pointer on the ground relay will then point to the red dot. The pointer can be seen through the window on the plate covering the relay (located in the main control compartment).
Large rigs are usually divided into two power systems for grounding purpose. They use two ground relays, either of which will shut down its position of the complete system. On smaller rigs, one relay is used to shut down the complete system.
When the ground relay trips, the power contactors are automatically opened and a light on the control panel comes on to signity the cause of the shut down. It is then necessary for the operator to return all throttles to the OFF position, reset the relay (push in the reset button) and by using assignment switches and throttles, pick up the generators and motors one at a time until he picks up the one in which the ground has occurred. If the ground is still on the system,the relay will operat again and shut down the equipment. A faulty circuit or machine can usually be isolated in this manner and operations can continue with the remaining machines.
The disconnect switch (normally the SPST toggle type) is located between the ground relay and the negative bus for two reasons:
1. It provides a means for testing to ground. With this switch open, the intentional ground is removed from the system and testing can be performed with a megohmmeter or other means.
2. It permits the driller to continue operation with a ground on the system by disconnecting the relay from the negative line. This way, he can get off bottom or perform any other urgent short-time operations even though a system ground exists. Limit such operations and remove the ground as soon as possible. Restore the reley switch to its normally closed operating position after removing the ground.
This grounding system will effectively minimize damage caused by flashover, it is merely necessary to clean up the commutator and then resume operations. The ground relay also serves to protect against cable faults and other insulation failures which might cause a fire or other damage. If the ground relay were removed by opening the cut-out switch while operating, these protective features would be lost.
PRINCIPAL CAUSES OF ELECTRICAL GROUNDS
Grounds are frequently caused by:
1. Insulation which has been mechanically damaged by wear, abrasion, cutting, etc., and insulation which has deteriorated by overheating
2. Leaking paths (across string paths, armature bands,brush-holder insulators, field-coil connections, etc.) Which have formed on insulation surfaces. These paths are accumulations of carbon, dirt, oil, or grease.
3. Moisture which permeates insulation and allows current to leak away.
4. Foreign objects, such as tools, which become conductors themselves.
LOCATING ELECTRICAL GROUNDS
To locate electrical grounds:
1. Return the throttles to OFF.
2. Reset the ground relay.
3. Assign main generators to the drilling motors one at a time (to pick up individual generator motor combinations) until the grounded power circuit combination is located.
4. Open the ground –relay cut-out switch.
5. Test insulation resistance of grounded circuit with a megohmmeter and record the reading
a. Open the cicuit connections at various convenient point.
b. Test separate part s of the grounded circuit with a megohmmeter . Look for signs of grounds in the main generator, the drilling motor, leads, cables, and other components.
Note: Insulation in poor condition will give readings at the lower and the scale. A Zero reading indicates a complete ground.
6. Reconnect the desired power circuit combinations and recheck the insulation resistance. If the ground has been corrected, the megohmmeter reading will be substantially higher than reading obtained in item
Note: It is generally necessary to keep the power circuit readings above one megohm to prevent serious trouble.
7. Before continuting operations, be sure to close the ground-relay cut-out switch.
