Case study 3

Poor Control Wiring within Switchboard

Location: At 400V 3-phase LVSB-01; Switchboard already turn-on

Fault & Consequence

Yellow phase control cable shorted to switchboard earthed metal frame while closing the fuse holder of outgoing indicating light after lamp replacement.

The phase to earth short circuit dissipated a substantial amount of heat energy creating a hefty spark causing visible flash marks on the panel (refer Pix.A).

Observation

The switchboard was shut down completely for investigation & prior to dismantling the din-reel (refer Pix.B); the insulation of yellow phase control cable had degraded leaving a trace of cut on the un-melted portion

Post Fault Synopsis Fault Type: Momentary Earth fault as the contact between yellow-phase to

earthed metal frame was only a short duration, i.e. once the electrician released his hand, the short circuit was opened.

Experimentation & Analysis: Direct acting for branch ACB of MLVSB, Ibranch = 2400A (1.5 x In) Earth loop impedance recorded at the pt of fault after supply resume, ZEFL= 0.19 Duration of fault was simulated & recorded with stop watch, TEF = 0.3sec Magnitude of fault current, IEF = 230V / 0.19 1211A

Since IEF < Ibranch, the branch ACB did not operate is technically correct. Though the external protection relay EF setting of 120A should clear IEF, the fault interval of TEF = 0.3sec is faster than the tripping time set, i.e. 0.5sec. Thus the EF relay did not operate either.

Root Cause

This incident could be fatal and unfortunately it was not detected during the insulation resistance test on L.V. busbars. The switchboard manufacturer had failed to ensure proper workmanship & quality control.

At LVSB-01 The protection relays & protective device at LVSB-01 did not see the fault as the wiring for incoming indicating lights were tapped from the line terminal before the ACB as shown in single line below. At MLVSB (Upper Stream) Both the branch ACB & incoming main ACB c/w external protective relays did not operate either.

Part of LVSB-01 Part of MLVSB

PowerGrid Incomer

OC Setting PMS 75%; TMS 0.05sec EF setting

OC Setting PMS 75%; TMS 0.1sec EF setting

Y-Phase to earth short circuit

5x300mm2 AL/X/S/P + 1x185mm2 CPC

2000/5A

2000AF/1600AT TPN ACB (50kA) DA setting 3200A

2000AF/1600AT TPN ACB

2000AF/1600AT TPN ACB (50kA)

2000/5A CL5P10 20VA

Fig.1 Single line representation on tripping incident