De-energizing an Electrical System under load

[EEAOC]

I would like to know what the precautions those are necessary in de-energizing electrical systems [medium/ low voltage switchgears and others] in terms of electrical safety. Can some one explain me the dangers involved if correct procedures were not followed?
Thank you in advance.

 

[RalphChristie]

There are not really any more dangers involved (or what I'm aware of) of switching systems off under load and switching systems off without load. There will be a little more stress on your contact-tips and diëlectric medium of your switchgear. But the switchgear is rated (or have to be rated) to break fault-currents that are much more worse than load current.

The thing to consider is the load itself and the consumer. If it is for instance a conveyor that is fully loaded, you can have problems to restart it again. It is more difficult for a motor to start under load than starting without load.

It is also uncomfortable for the consumer if the power is suddenly cut without warning.



Regards

Ralph

 

[tinfoil]

I have encountered more than one meaning of the word 'de-energize'

Do you define 'de-energizing' to mean
"removing all active sources of energy"
or to mean
"removing all active sources of energy and solidly grounding the equipment"

 

[Sargardani]

If there were risks then breakers won't have been designed to trip!

Sarg

 

[alehman]

When operating any power switch or circuit breaker it is always good practice to stand to the side so if something goes wrong you won't get hit in the face with the possible flash or flying parts.

If it is a medium voltage substation switch or distribution cutout, be sure it is rated as a load-break device.

Make sure whatever equipment is being de-energized won't cause hazards by stopping.

 

[rbalex]

You may find NFPA 70E-2004, "Electrical Safety in the Workplace," useful. It contains a broad range of safety related work practice requirements including opening circuits under load.

 

[skm]

Absolutely no danger involved as long as the disconnectin equipment is rated for opening under rated load condition. If the disconnectiong equipment used is breaker you can simply take the feeder off. In case of disconnector / isolator please check its rating w.r.t. on load and off load operation.

The above procedure is w.r.t simple disconnection. Should you require to maintain the system after disconnection please follow the necessary guidelines w.r.t earthing of the equipment and other safety measures as required per local guidelines.

Cheers

 

[alehman]

I take exception to skm's statement that there is "Absolutely no danger..."

Yes, if the equipment works as it is designed and it is applied correctly, the danger is minimal. But there are no guarantees in life. Equipment sometimes fails and engineering errors result in applications outside of ratings. When circuit breakers and switches fail, it is nearly always when they are being operated. Available fault current sometimes changes after installation. Also, it is not unusual for a small amount of flash to escape molded case and air circuit breakers when interrupting fault current. I would rather not have that in my face.

 

[RonShap]

As mentioned by rbalex, look at NFPA 70E. You are working "on or near" an energized circuit. Operate the breaker or disconnect with the enclosure closed.

 

[TrainerBob]

Without knowing if you mean "turning off a switch" or "unbolting a bus bar", it's hard to answer. But here is the spectrum that I've seen:
An electrician on an aircraft carrier had a bus bar come disconnected in a 480V switchgear, with the resultant arc causing a fireball and molten metal killing the lad.
An operator at a power plant opened a 345KV (yup, KV) disconnect under load-the disconnect meant as an isolation device, not load interrupting-and drew a VERY impressive arc, but no damage.
So to answer your last, the dangers if procedures are not followed can be severe-arcs, sparks and injuries.

 

[robertb]

Under some possible failure conditions, parts of the electrical equipment may maintain a dangerous electrical charge even after the system has been disconnected from power for several minutes. If wiring is ever disconnected during operation there is an especially high risk of equipment maintaining a charge. If there is reason to suspect that danger is present, do not come in contact with any pieces of the equipment, including wiring, for several minutes.