Line Isolation Monitor

[irtiq7]

Hi Everyone,

I am trying to design a simple Line Isolation Monitor that is capable of detecting a single line fault from either line to ground. But so far I have no clue how to begin. Could some one help me out please!

Thanks

[Max]

A Line Isolation Monitor (LIM) monitors the integrity of an isolated power system. Isolation transformers produce non-grounded power sources. All AC wiring and AC operated electrical devices have the potential for direct resisting capacitance, or inductive leakage currents to occur via the casing of most electrical devices, thus there are small leakage currents. The LIM, also known as the ground fault detector, indicates (in milliamperes) the total leakage from the system.

LIM readings of + milliamps does not mean that current is actually flowing, but indicates how much current would flow in the event of a second fault. It is usually caused by someone touching the "non-earthed" side of the circuit. Thus the alarm does not mean that there is a hazardous situation, however there is potential for it, should a second fault occur.

In the event the LIM reads zero amperes with no leakage current—and if one side of the electrical current becomes "relatively" grounded, touching the current at the ungrounded side, it allows the current to make a complete path, thus causing an electric shock.

Other important terminology includes:

• Microschock: Current delivered directly to the heart. A current of 50 microamps can cause V-Fib. (Depending on current density.)
• Macroshock: Current delivered through the skin or tissue remote from the heart.
  • Macroshock of 0.1-2.5 Amps is required to cause heart fibrillation.
  • 6.0 Amps causes sustained myocardial contraction followed by normal rhythm, with temporary resp. paralysis [usually if J>100 mA/cm].
  • 8-20 milliamps causes muscle contraction. ("Let go” current level.)
  • 1-2 milliamps causes pain.

The LIM is set to trigger an alarm at 2 or 5 milliamps, depending on the age and type of system. (5 mAmps is current standard.) Thus it will not warn of microshock range leakage current, so equipment coming in direct contact with the heart, (i.e.: ECG Monitor when placing a central venous catheter) should be pre-checked for leakage current during routine maintenance.

[irtiq7]

Thanks Max for a detail explanation but my question was, what components do I need to build one?

[Magneto]

Dear Friend ,

In power wire , which come to your home endup with a transfomer , which should have situated near your house area.
You are getting power through this transfomer. In Sri Lanka , Neutral line has grounded in this transfomer in the place where it situated.
There are two reasons for this.

1. For protection 2, maintain zero potential in neutral

So let say if your Live wire accidently touch the earth , then there create a earth leackage path from your house to transfomer. So this will lead to flow high current in this earch leakage path from you live wire.

So by detecting this high current , you can make simple line isolation monitor. So for this you have to monitor the current flowing in such a occation. Ideal equipment for this is a current transfomer. you have to connect the live wire to the primary of current transfomer and according to the current flowing through primary , there is a varying AC voltage generating in secodary of current transfomer.

This is your signal , which you have to analyse and take the dission where there is ground sitation occor or not.
You can analyse this AC signal easily by converting it to DC and feed it to a comparator or to a small microcontroller.
If you are using a microcontroller you have to use a analog to digital converter of the microcontroller. Make sure
first make the signal voltage to be compatible with microcontroller signal voltage level. You can easily do this by using
a voltage devider with two resistors.

Regards,
Magneto

[irtiq7]

Thanks Magneto!!