High Voltage Safe Working Procedures
3.8 Earthing Down
The higher levels of insulation resistance required on high voltage cabling leads to higher values of insulation capacitance and greater stored energy by the formula:
Energy stored (W) joules = (Capacitance x Voltage²) 2
This stored high energy can be fatal to the person working on the HV conductor. Earthing down is very important when working with high voltage systems. It is important to ensure that any stored electrical energy in the equipment insulation (after isolation) is safely discharged to the earth.
There are basically two ways to earth down a HV switchgear:
1. Circuit Main Earth (CME)
An earth connection is applied to ensure that the apparatus safe to work on, before a permit to work or sanction for test is issued and which is mentioned in the document.
Once the circuit breaker has been racked out, all three phase conductors of an incoming or outgoing feeder cable earth connection from the earth is connected in the cable compartment of the switchgear.
The earthing switch is mechanically interlocked which also ensures that the circuit breaker can be racked back in only when the circuit's earth connection has been removed also can be provided with locking facilities (e.g., the provision of padlocks).
Cable Compartment
Disconnector / earthing Switch
Circuit Breaker Compartment
Bus BarCompartment
Figure 3.5
An Earthing Switch in a Cable Compartment
High Voltage Safe Working Procedures
Figure 3.6
An Earthing Lever Slot in Front of the Switch Gear
Figure 3.7
An Earthing Switch in the Cable Compartment
3.8.1 Additional Earth (Portable Ground Earth Rod Set with Earthing Wire and Clamp) 3.8.1.1 Grounding Sticks
Grounding sticks are long, non-conductive sticks with metallic ends. The ends have a solid connection to large flat copper braid or large gauge wire, with a short run to a good earth / grounding point. The grounding stick can be used to ground the leads conductors of any supply conductor after the circuit is de-energized to ensure that the stored energy in the HV conductor is discharged to the hull of the ship.
An earth connection is applied to an apparatus after the application of a circuit main earth (CME), that is normally applied at the point of work, if it is not already fitted with a CME (Bus Bar).
It is a safe practice to use a portable earthing kit at the point of work as in the case of a generator / transformer / VFD, etc., as a circuit main earth does not complete the earthing of the section at the secondary of a transformer or at the output terminal from the starter of a variable frequency drive as shown in Figure 3.9.
Chapter 3
Figure 3.8 Grounding Sticks
Figure 3.9
Earthing Down a VFD
A common earthing wire is to be connected to the earth point first and then to the HV conductor before starting the work and to be the last to be removed from the common earth after the work is finished.
Earthing down is with the help of portable leads that complete the earthing arrangement; they also serve to indicate that earthing down has been done.
Figure 3.10 – Portable Earthing Kit to work in a VFD
High Voltage Safe Working Procedures
3.8.2 Bus Bar Earthing
Bus bars must be isolated from all possible electrical sources whenever work on a section of it is to be carried out. This includes generator incoming lines, bus couplers, section breakers and all transformers in that bus bar section.
The bus bars are then connected and earthed down with the help of portable leads that complete the earthing arrangement.
3.8.3 Safety Lock
Safety key locks are used to secure the points of isolation and earth circuits so that all the interlocks in the isolation procedure are adhered to.
3.8.4 Key Safe
A device for the safe retention of keys that are used to lock the means of isolation, earthing or other safety devices.
Figure 3.11 – A Safety Lock 3.8.5 Dead
A circuit is considered dead or at zero potential when it is disconnected from all sources of electrical energy.
Proving a circuit dead is the most important operation prior to HV work and a part of the HV Permit to Work procedure. It is done by a voltage tester which can be used from a safe distance.
Chapter 3
3.8.6 Locking Off
A shutter assembly is installed in switchgear cabinets and is designed for the protection of the attending personnel against contact with live parts of the main circui fixed contacts installed in bushings. All spout (orifice) shutters not required for immediate work or operations are to be locked shut.
Exception: On certain types of switchgear, access to the shutters is restricted whilst the circuit breaker is still in the cubicle. Under these circumstances, it is acceptable to lock either the cubical door or the racking mechanism, whichever is appropriate, which must prevent further withdrawal of the circuit breaker, so long as the circuit breaker has been withdrawn from its normal operating position.
Figure 3.13 – A Shutter Assembly 3.8.7 Live
A circuit or component is deemed to be alive when it is (electrically) charged from an electrical power source.
3.8.8 Withdrawn Apparatus
High voltage apparatus which has been isolated and removed from its normal operating position may be worked upon, provided that:
a) It has been discharged.
b) Prevented (by barriers and locking arrangements) from being restored to a live state.
c) Access to high voltage conductors on the switchboard is prevented.
d) Switching and isolation procedure on a marine HV system is carried out.
e) Tests of insulation and resistance on high voltage equipment have been performed.
The safety shutter will be closed during withdrawal of the circuit breaker so that the HV terminal of bus bar is not exposed.