Open secondary circuit of a current transformer
It is well known that an open secondary circuit of a loaded current transformer would cause a very rapid and large increase in voltage on the secondary side that could lead to arc formation and possible explosion.
Working on the secondary circuit of a current transformer that carries a high current (e.g. 800 A) on the primary side is definitely a very dangerous job, because there is always a risk that a loose connection on the secondary side might cause the secondary circuit to open, thereby forming an arc almost instantly.
The formed arc could burn many of the low voltage components in the low voltage circuits on the secondary side and if the arc is formed while an operator is working on the low voltage compartment then he becomes prone to great danger.
Disconnected earthing bar
One of the very serious mistakes that occur, when working with medium voltage switchgear is the case of a working while the earthing bar is disconnected.
Sometimes there are installation mistakes, and in one of those cases, the earthing bar connection between two cubicles might be loose, making some of the cubicles totally disconnected from the earthing system of the entire switchgear, and this becomes a great hazard for a technician doing maintenance on one of the disconnected cubicles, because he becomes exposed to several dangers, for which the least is electrostatic discharges.
Figure 1: connection point between the earthing bar of two neighbouring cubicles
Circuit-breaker with worn contacts
Poor maintenance leads to malfunctioning equipment, and malfunctioning equipment can cause serious injuries to people. Circuit-breakers need a lot of maintenance, since after some time parts of them wear out.
When one of the circuit breaker contact rings wears out, one should talk about the hazard of arc formation between the fixed contacts of the switchgear and the movable contacts of the withdrawable circuit-breaker. This is quite dangerous because if these two contacts do not fit like a lock and key, the arc formed could create a force that would shoot the circuit-breaker outwards towards the operator, while struggling to fix it in the right position.
Capacitor discharge
Capacitor discharges in medium-voltage are a very serious matter, one could simply imagine the amount of energy stored in a 11 kV capacitor. It is quite common to find capacitors around motors, like water pumping stations, since they are often used to improve the power factor of the power supply to the motor.
The best advice regarding working with capacitors is to have a very good earthing system to discharge them before working on the capacitors and a calibrated, long voltage tester to make sure there is no voltage across the terminals of the capacitors. The capacitor positive terminal should be constantly earthed while working on it.
Power from another source
Figure 2: Example of a ring topology of a power system with four ring main units
When working with ring main units it is quite common to have power from different sources.
When working with a cubicle in of the main ring units the other cubicle in the adjacent ring main unit, which could be far away can be energized and deliver power to the cubicle where the engineer or technician is working, so care must be taken that both sides are isolated before work.
Working with live switchgear
Some service jobs require working with live switchgear, when the plant cannot be shut down for any reason, like in the oil and gas sector, where every minute of production counts.
When working with live switchgear, proximity from the apparatus becomes very important, one should not get too close to the equipment because of the strong electric fields associated with medium-voltage switchgear.
As an example, the engineer could place himself at a sfe distance from a live apparatus, but unconsciously point his finger towards the switchgear, as if explaining to the customer, thereby closing the distance with the apparatus and risking electrocution by entering the electric field.
Conclusion
In this article, some of the popular deadly mistakes that could happen while working with medium-voltage switchgear have been discussed.
The article goes through six main dangerous mistakes, namely (1) an open secondary circuit of a current transformer, (2) a disconnected earthing bar, (3) working with circuit-breakers with worn contacts, (4) working with capacitors, (5) the hazard of power from another source and (6) working with live switchgear.
Thanks for reading,
Ahmed H.
Do you think that there are others deadly mistakes? Please share your thoughts in the comments below.
Short circuited potential transformers.