PORTABLE TEMPORARY GROUNDINGS (PTG)

PORTABLE TEMPORARY GROUNDINGS (PTG)

              Purposes or Advantages of PTGs

1.                  PTGs are positive proof of electrical isolation.
2.                  PTGs drain e.s. and e.m induced potentials.
3.                  PTGs guard against the accidental magnetization of circuit.
4.                  PTGs provide bonding.

                 Grounding / Bonding Differentiation  

Grounding

                                  Grounding provides low resistance parallel path and hence by passes the worker if and only if worker is standing on a grounding mat. The current flowing through grounds operates earth fault relays which then trips the breaker and hence equipment is isolated. Thus we can say that “Grounding Clears the Fault and Provides Bonding for the Worker”.

Bonding

                                   By bonding we mean to connect all the metallic parts in the work area to station grounds in order to maintain them at same potential i.e. ground potential so that there is no danger of appearing of potential difference within the reach of worker feet or hands in case of fault / inductions etc. Hence we can say that bonding saves the worker.

Examples of Bonding 

                                     In case of ground fault, ground voltage rises and it decreases as we move away from ground rod, as shown in diagram, due to earth resistance. In the diagram shown person A and person B both are standing on earth or mat which is not grounded. In case of ground fault both will be standing at different  potentials than the ground potential due to absence of bonding. When person A will touch the tower / person B will take steps, both will be subjected to electric hazard due to existence of potential difference between hand and feet of person A and between feet of person B.
            In case of ground fault the potential experienced by a person standing on earth or ungrounded mat while touching the tower is called Touch Potential.
            In case of ground fault, the potential experienced by a person between his feet when taking step in the ground fault vicinity is called Step Potential.
These both potentials can be eliminated by grounding the mat on which person A and person B are standing i.e. by providing bonding.                                                         
 

                   Important points when applying PTGs

1.                  Approved sticks must be used when applying / removing PTGs.
2.                  In damp weather rubber gloves must be worn when applying grounds and grounding stick must be held away form the body.
3.                  Grounding cable must be in good condition and of proper size.
4.                  When applying PTGs first apply ground at ground end and then at line end. When removing PTGs first remove from line end and then from ground end. Ground connection at ground end must be tight and for proper tightening ground cable should be secured by rope.
5.                  Eye protection must be worn when applying / removing PTGs.
6.                  Line must be teased before applying PTGs. The hearing of buzzing sound when metallic head of grounding stick is brought near a live circuit is called line teasing.
7.                  When applying ground on a line, if line is found energized then ground must be applied so that ground fault relay trips the circuit. If in this case ground is removed, heavy arc will be established causing power arc hazard.
8.                  Ground clamps must be in good condition and of proper size.
9.                  If the weight of ground cable is too much then two person should do the job.
10.             PTGs must be applied to all three phases even if work is only on one phase due to the following reasons.
i.                    Worker may forget that the other phases are not grounded. The other phases may have induced voltages and when he will approach the other phases or comes in contact with other phases accidentally, he will be subjected to electric hazard.
ii.                 3-phase tripping is faster than 1-phase tripping so fault will be cleared soon if all the 3-phases are grounded. Moreover, cable rating is only for 25 cycles and if only one phase is grounded it may take long time to clear the fault and hence the cable may burn away.  

   Grounding of Dead Circuit near a Live Circuit

In the diagram shown very high current may circulate in the loop. After the completion of work when PTGs at one end will be removed first, arc will be produced due to current interruption causing power arc hazard. To prevent this, either use only PTGs at one end or keep breaker open or keep grounds as close as possible.