To understand this first of all we have to know the DC Grounding Methods which are
- Floating from ground system
- DC Ground detection system
1. Floating from Ground System:
Stationary battery and dc power systems used in switchgear and control applications are typically designed and operated as a floating from ground system which means that there is no intentional low resistance or solid connection to ground from either the positive polarity or negative polarity of the dc system.
In this case with no unintentional ground paths should have zero potential between the positive bus to ground and the negative bus to ground.
This type of floated from ground dc system is dangerous to personnel. A low-resistance ground on the positive bus can result in an electrical shock if personnel were to accidentally touch the negative bus. Likewise, a low-resistance ground on the negative bus can result in an electrical shock if personnel were to accidentally touch the positive bus.
Why Doesn’t Tester Glow?
In this case if we touch the tester to either positive or negative poles of MCB, no current flows as it is grounded at only one point, through the human body and there is no return path.
2. DC Ground Detection System:
The most common types of ground detection systems are balanced resistor ground detectors. It has a medium resistance connected between the positive polarity and ground, and an equal resistance connected between the negative polarity and ground.
This arrangement is made inside the Earth fault Relay. Here we will get equal amount of potentials for positive and negative poles with respect to earth.
Now if we place the tester on either positive or negative poles of MCB the unbalance in earthing causes it should glow.
Why Doesn’t Tester Glow?
In any case if we bypass the earth fault relay or disconnect its terminals then the grounding no more exists and it is same as floating ground system, so tester will not glow.