Safety Operation of Medium Voltage Switchgears

Hazards of operating on electrical equipment

Medium-voltage switchgear. Eaton's medium-voltage switchgear provides centralized control and protection of medium-voltage power equipment and circuits in industrial, commercial, and utility installations involving generators, motors, feeder circuits, and transmission and distribution lines. There are five kinds of hazards related to the operation of electrical equipment.

1. Electrical shock
2. Electrical burns
3. Fire and explosion
4. Heat build up
5. Mechanical hazards
6. Electrical shock

Because there may be a serious threat of flashover, shock, arcing, etc. when working in a medium voltage environment, decrease voltages tend to be handled with less admire. It need to be remembered that a voltage as low as 50 V, with a contemporary of ± 30 mA, can reason asphyxia and/or heart muscle fibrillation. The biggest hazard of strength is that live and dead apparatus are indistinguishable from each other. Therefore, equipment ought to in no way be assumed lifeless, constantly presume it's far alive. This can simplest be ascertained by checking out with the appropriate accepted tester, or by using the presence of a visible earth connection. Electricity can bounce gaps, which means that that it isn't even important to the touch a medium voltage conductor so as to get hurt. Merely approaching too near may have fatal results, therefore, it's far important to preserve close proximity distances.

1. Identify the proper substation/MSS etc.
2. Identify the best circuit breaker/ isolator.
3. Switch – open the circuit breaker/ isolator.
4. Isolate the circuit breaker with the aid of racking out/ down.
5. Test – the use of accepted voltage detector.
6. Earth – connect correctly to earth the use of correct method.

Mini sub stations (MSS) and ring fundamental panels (RMP) are opened manually the usage of the ideal working handle. The “T” off to the transformer may be furnished with a circuit breaker or fuses and opened through pushing a button on the the front of the unit. They also can be provided with an electrical remote operating device, similar to a circuit breaker. Isolate Isolution mains bodily disconnecting the equipment from all feasible resources of electrical potential by using:

1. Opening and/or putting off of fuses.
2. Opening of links.
3. Withdrawal of truck type switchgear.
4. Lock off and apply risk tags.

Security at factors of isolation:

All points of isolation ought to be locked off through the application of a non-public lock to prevent inadvertent operation of the mains or equipment. Danger tags: The use of threat tags is an essential part of the isolation process. Danger tags need to be carried out on the transfer or control equipment which has been remoted, with the intention to enable men and women to paintings on electrical equipment or conductors safely, by way of notifying people that paintings is being conducted on that piece of apparatus. 

Authorised character: 

A individual advocated via the electrical engineer or his nominee and appointed in writing with the aid of the designated individual to perform switching, isolating, testing and earthing on electric mains and apparatus, in liaison with and beneath the practise of a manipulate officer and to trouble work permits in recognize of such mains and equipment. 

Isolation technique 

The authorised person shall perform the necessary isolations, using personal locks,  onsistent with the policies and regulations. Thereafter, perform safety tests the usage of an appropriate authorised tester to make sure that the mains and/or equipment are useless and apply risk tags at all control points.

Control factor:

A function at the system where a major and/or apparatus may be switched, isolated, and earthed. When isolating, the equipment should be isolated, from all viable resources of electricity, not just energy. Therefore, we need to keep in mind the subsequent sources of hazardous strength and hazardous substances.

Hazardous strength:

Electrical, pneumatic, hydraulic, stored (springs, batteries), ability (via distinctive feature of position), heat (hot water, steam), radiation.

Hazardous substances:

Gases, vapours, liquids, dusts with the ability to purpose injury or illness, e.G. Toxic, corrosive, flammable. All plant and system need to have written methods for isolation; these approaches will set out a step by step account of the way the gadget, plant or system is to be remoted and kept secure. In the case of electrical isolation, a check for voltage should be performed with the ideal approved tester, to make sure that the mains and/or equipment are dead.

Summary

The authorised character plays the isolation in line with the regulations and regulations. Control factors need to be locked off the usage of private locks. Danger tags ought to be implemented to all factors of isolation.

The locking of manipulate factors and alertness of risk tags is crucial for a safe isolation, and isolation will not be deemed complete till those necessities are met.

Locking off of stay shutters

All stay shutters ought to be locked off with the personal lock. Cable shutters must be classed as stay shutters, as the cable might be returned fed, e.G. Open factor on a hoop, consequently, both bus bar and cable shutters should be locked off.

Test

Before applying earthing device, the conductor have to first be tested to prove it lifeless. Before the use of any approved medium voltage system it have to be physically inspected for defects. When testing medium voltage the 3 factor take a look at have to be adopted, test the tester on a known stay source or a check box furnished with the aid of the manufacturer, test all 3 levels and retest the tester.

There are several sorts of voltage testers on the market and they all have particular uses:

Voltage detector (live tester): 

Used for testing the presence of voltage. A live tester is one that has to the touch the conductor underneath take a look at if you want to decide if it's far stay or useless (it is recommended that one with audible and visible annunciation is used). These live testers are synthetic in two distinct types – S and L. S kind to be used on switchgear. L type for use on overhead lines.

Phase comparator/phasing sticks (live tester):

Used to test that circuits are in segment with every other. A phase comparator have to be used for phase contrast and now not voltage detection.

Phasing in of ring feeds

It is vital to segment in medium voltage system prior to energising cable circuits, to make certain the appropriate segment rotation whilst cable systems are maintained and extended. Electrical phasing have to be carried out while:

1. New system is installed, which necessitates breaking into a ring feed, e.G. New substations.
2. After the repair to any cable which forms part of a hoop feed.
3. Whenever a cable which forms a part of a ring feed is terminated.
4. An existing cable (to be again to service) is being joined and/or terminated.

Voltage detector (proximity tester):

Used on overhead lines. This tester does now not must come into contact with the conductor below take a look at to determine if it's far live or dead, it detects the magnetic field and consequently best works on naked and unscreened conductors.

There are no test facilities to test the cable is useless before earthing on an MSS/RMP, however, maximum modern units are supplied with LEDs, indicating if the cable is live or useless and these should be used as a guideline whilst operating. They additionally have the ability for doing electrical phasing on the front of the unit, using a multimeter. MSS/RMPs do have take a look at points, however these are for checking out the cable as soon as it's been earthed and consequently get admission to can only be won to them once the cable has been earthed.

Earthed

Connected to the overall mass of earth in such a way as to make sure always an immediate safe discharge of power. There are several special forms of earthing methods: fundamental earthing, earthing carriage, earthing truck, and transportable earths. Integral earthing is designed into the circuit breaker and no outside attachments should be implemented to the circuit in order to earth it.

Earthing bus bars

When earthing a bus bar all viable assets of deliver must be isolated. Therefore:

All far flung ends ought to be switched, isolated, locked off and threat tags carried out.

All nearby circuit breakers must be switched, remoted, locked off and hazard tags applied.

Test with an accepted voltage detector.

Apply no less than one earth. Portable earthing device A p p l i c a t i o n o f p o r t a b l e e a rt h i n g system:

1. The mains and/or equipment to be earthed ought to be tested with an authorised voltage detector to ensure that it's far dead.
2. Check the condition of the portable earths
3. Securely connect the flexible cause an earth bar or earth conductor FIRST.
4. Using an earth or hyperlink stick touch every segment of the mains and/or apparatus to be earthed.
5. Clamp onto every section.
6. When casting off the portable earths theearth connection need to be eliminated last.
7. Care must be taken to make certain exact tight connections. It is the authorised character’s responsibility to ensure that all portable earths are removed earlier than energising the circuit.

Interlocs

All circuit breakers and MSS/RMPs have mechanical interlocks, for example, you can't rack a circuit breaker out if it's miles closed, you'll no longer have the ability to region an earth on an RMP unless it's been opened first. However, they're no longer interlocked with each other and it is feasible to earth a live cable. To this cease, the ideal oprating method ought to be accompanied and the authorised individual need to ask himself, earlier than operating, what impact will the operation I perform have at the rest of the device – for every action there's a reaction! The following is a listing of a number of the matters to remember earlier than working:

Switch (open)

Will any deliver be misplaced? If so, can I reroute the supply (shift open point on the ring, or shift load onto any other circuit).

1. If load has been shared with some other circuit, make sure that that circuit can convey the blended load.
2. If load has been shared with any other circuit, ensure that that circuit has picked up load before starting the circuit breaker.
3. Once the circuit breaker has been opened, take a look at the other circuit has picked up the full load.
4. Can the circuit breaker be opened remotely, if not what PPE is required?
5. On a MSS/RMP the LEDs may be used to see if supply is lost to different circuits and to test if the circuit is lifeless.

Isolate

Have all 3 phases on the circuit breaker opened?

Before racking out/down a circuit breaker, it is right practice to test all three phases are now not drawing load, this could be done via the usage of the ammeter selector switch, if one is fitted.

Test

Is the tester suitable to be used at the system? E.G. correct kind and voltage rating.

Is the tester in exact circumstance?

There are no check facilities (NTF) on a MSS/RMP prior to earthing.

Earth

Is the opposite facet of the circuit opened and isolated or earthed?

When reinstating the circuit

Remove earths

There is no electrical tripping in the earth function on a circuit breaker, so the circuit breaker needs to be tripped manually.

Close

Has the earth been eliminated on the faraway stop?

1. Has the far off end been made safe? E.G. Has the area been checked after paintings that there no tools were left inside the operating place. Have employees been warned it's miles now not safe to work within the place. Have portable working earths been eliminated etc.
2. Has the remote cease been secured? E.G. Has it been isolated and locked off?

Work allows

Written authorisation for work to be accomplished on electrical mains and/or apparatus. Once the circuit has been made safe for paintings as above, the authorised person will difficulty a medium voltage work permit for work to be performed on the remoted mains and/ or equipment. It is crucial when issuing a work permit the following interaction with the character in fee is executed:

1. Give explicit instructions to the individual in price of the paintings regarding the paintings to be performed and the secure limits of the workplace and make certain that this is understood.
2. Show the person in price the manage points and permit him/her to location their non-public lock at the manage point.
3. Earth or show the mains/equipment dead on the worksite.

When receiving the work permit returned on the place of job from the person in fee, the authorised person is answerable for acquiring confirmation that each one persons are clear of the mains and/or equipment and that each one non-public running earths implemented at some point of the work were removed.