Insulation monitoring device or system for LV switchgear

[sykimk]

Have you anyone else heard about insulation monitoring system according to IEC standards?
One of manufacturer insists this device must be installed in each bus section of switchgear for TT & IT grounding system as per IEC.

If you have an experience to use this device, please let me know the relevant sections of IEC and the purpose and operating scheme of it.

Thanks in advance.

 

[ruggedscot]

IEC 61557-6 Ed. 1.0 b:1997

Title: Electrical safety in low voltage distribution systems up to 1000 V a.c. and 1500 V d.c. - Equipment for testing, measuring or monitoring of protective measures - Part 6: Residual current devices (RCD) in TT and TN systems

Abstract: Specifies requirements for measuring equipment applied to the testing of the effectiveness of protective measures by regular disconnections of residual current protective devices (RCD) in TT and TN systems.

The method used by Norway is known as the IT System and the only things grounded are the metal casings and parts at the load, which share a common ground line NOT bonded to the neutral. Norway uses it particularly because the earth has very low conductivity and other countries use it around hospitals or where human safety is emphasized. There are basically two other systems; the one used by the U.S. is the TN System in which all neutrals, casings, and metal parts are bonded to the earth. Essentially, this system forces the earth to share the common point of the distribution system.

The pros of the TN System are that it's very easy for protection devices to detect faults and the earth can be used as a (supposedly) temporary conductor to maintain the continuity of distribution. The cons you know better than me - the earth is now "energized" directly at the distribution's potential and its wide use as a conductor is becoming a hazard for people and animals. As no devices are created for separating the service neutral from the load neutral at a high-voltage fault, all the loads are affected by it. Also, the quality of the signal is diminishing as more demanding and sensitive equipment is used in the continent and my island. A measure of improvement is the use of the fourth wire at the loads, where this wire goes directly to the neutral of the service and provides a better, less dangerous path for the fault currents.

The last system is the TT System where the supplies' neutrals are grounded and only the casings and metal parts of the load are grounded. This provides for better detection of low-power ground faults and, although the earth is still used as a conductor for these faults, their paths are less and accidents are fewer. This is the most widely used grounding system in the world, a product of its versatility and reasonable cost of protection devices.

Id say an insulation monitoring device is something that watches for any leakage to ground and sends a warning to the user or else trips the system. Some systems operate earth free and require to be monitored so that if one goes down to earth then it can alert. In your case it sounds very much like there will be a trip of supply as both the IT and TT systems employ earthing and thus any fault to ground will cause a problem and require to be flagged up and tip out the supply.

5mA
30mA
100mA being the usual thresholds

Rugged

 

[tulum]

SYKMIK,

I don't have much experience with insulation monitors, however more amd more customers are requesting them.

What I have found is that Bender is one of the world leaders in insulation monitors. Their engineering people have a wealth of knowledge on the subject. Give them a call.

Bender even won an Emmy (or Grammy whichever it is) for their work on the movie Titanic (They did all the ground fault protection etc.)

Here is one link....

http://www.bender-de.com/

http://www/bendergmbh/bendergmbh.asp?lng=1033

Regards,
TULUM

 

[jraef]

This outfit makes them for motors, I don't know about entire systems though.

http://www.msegroup.net/prod-ms.htm

"Venditori de oleum-vipera non vigere excordis populi"

 

[mc5w]

Hi ruggedscot,

Can you tell me anything more about electrical systems in Norway?

1. Is the premises' equipment ground bonded to the case of the utility transformer and other premises' equipment ground? Starting with 2002 National Electrical Code this is the specification for ungrounded secondary voltages in the U.S.A. - you do not want to have a ground fault in one premises flowing to the other ground fault through the earth, water mains, or an adjacent grounded lighting service.

2. Are the secondary voltages usually ungrounded 3 wire 3 phase? Or, do they use some form of resistance grounding?

3. Are houses supplied with ungrounded 2 wire single phase or 3 wire 3 phase?

4. Are there any solidly grounded systems where the neutral bond in only done at the utility supply transformer and then seperate neutral and equipment grounding conductors run along with the hot wires to each service? Some farms here is the US have to be wired this way because of bad soil conductivity or to reduce tingle voltages in milking parlors. Unlike rabbits and rottweilers cows do not like the Cow Voltmeter Method.

5. How is detection and disconnection of ground faults done? Is the method strictly by ground detectors with manual disconnection of the ground fault or has somebody figured reliable automatic relaying for residential and commercial circuit breakers?

6. How would they go about getting Ground Fault Circuit Interrupter protection to work in bathrooms and so forth? GFCI requires a solidly grounded or resistance grounded system to work. Ungrounded systems can deliver a nasty electric shock if the system capacitance is high enough.

Mike Cole, mc5w@earthlink.net