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Designing a line isolation monitor for under water device

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gaminid

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Hello,

I have to design a line isolation monitor (LIM) for a underwater device which sits at a depth of about 500 feet. The voltage supply for this device is 400VDC and supplied via a cable. Here are my initial thoughts.

1. If it were 400VAC I could have used current transformers on Live and Neutral ground loop to detect any leakage current (by monitoring the secondary side of the transformer). But since this is 400VDC may be I will have to build a DC-DC converter where the the leakage current is the primary DC supply.

2. Do I need to worry about transient currents damaging the LIM electronics in case if the cable is broken suddenly?

Any advice/suggestion/guidance is greatly appreciate it.

Thanks

Gamini
 

Basically an insulation monitoring device (also called RCD residual current device when used in mains power supply) doesn't necessarily need a transformer respectively an insulated supply circuit. A differential current sensor would be sufficient to detect a leakage. A DC/DC converter however offer the advantage, that you can still operate the device in case of a minor leakage without fearing electrolytical corrosion. And by applying a test voltage, you can detect shorts of circuit ground to earth.
 

FvM: Many thanks for your prompt reply.

I did not understand electrolytical corrosion and the advantage of using DC/DC converter.

Thanks

Gamini
 

Difference between LIM and RCD.....

The Line Isolation Monitor (LIM) measures the total leakage impedance to ground in an isolated (ungrounded) AC power system.

An insulation monitoring device monitors the ungrounded system between an active phase conductor and earth. It is intended to give an alert(light and sound) or disconnect the power supply when the impedance between the two conductors drops below a set value.

A Residual Current Device is a generic term covering both RCCBs and RCBOs.

A Residual-Current Circuit Breaker (RCCB) is an electrical wiring device that disconnects a circuit whenever it detects that the electric current is not balanced between the energized conductor and the return neutral conductor. Such an imbalance may indicate current leakage through the body of a person who is grounded and accidentally touching the energized part of the circuit.
A Residual Current Circuit Breaker with Overload protection (RCBO) combines the functions of overcurrent protection and leakage detection.
 

Thanks for the clear definition Kak111. Could you give me any direction where to find a sample design or some book reference (or any other reading material) for LIM. Do I need to worry about the cable capacitance (about 500' long)? I couldn't anything that will help me on the internet yet....

Thanks

Gamini
 

Here is what a cable test guy is doing for isolation tests **broken link removed**. Is it needed to test the isolation during normal operation and what kind of cable is it? Another question is how many lines have to be tested? Only based on this a isolation test circuit test strategy could be defined.

Enjoy your design work.
 

Many thanks kak111. your google search gave me a lot of examples. Thanks again...
 

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