Insulation Monitoring System
Electrical safety for unearthed power suppliers:
•A-ISOMETER®
•Insulation monitoring devices
•Insulations fault location devices
The IT system:
No live conductor of the system has a conductive, low-resistance connection to the equipotential bonding system (earth).
Design of IT system:
The IT system is supplied by an independent supply source like
Distribution transformer for industrial purposes
Control transformer
Generator, power set
Battery
UPS converter
Solar panel
Advantages of an IT system :
Increased Economic Efficiency
Increased Operational Efficiency
Optimized maintenance
Extensive Protection against Fire
Enhancement of Accident
Higher Permissible Earthing Resistance
Insulation fault in an IT system:-
In IT systems (standard-compliant term for unearthed systems) the first fault does not have any negative influence.
The first fault does not cause unwanted system shutdowns.
A double insulation fault at different conductors is the same as a short-circuit.In this case, fuses are protective devices
Insulation monitoring device:
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, usually 50 kO.
Working principle:
Insulation Monitors superimposes a measuring signal, generated by signal generator G. On the occurrence of an incipient insulation fault, the measuring circuit between system and earth closed via the fault RF causing a voltage drop across the measuring resistance Rm, which is processed and evaluated by electronic circuitry. If the voltage drop exceeds the set value proportional to insulation resistance, an alarm is given. The signal has a DC component which is used to charge the system capacitance, thereby ensuring that the signal goes through the incipient fault. The nature of the measuring signal is very important as it should not affect operation of devices such as switched mode power supplies, computers, variable frequency drives etc. Further, the harmonics and other disturbances created by these devices, presence of filters connected to the system should not affect the measurement of the insulation fault.
The insulation monitor provides in advance, information for effective preventive maintenance thereby ensuring the availability of power supplies. It also prevents shock hazards in small and medium size low voltage power supply and distribution system.
Factors inflencing measuring technique:
DC voltages in the system
High system leakage capacitance
Variable, low frequencies
Active measuring principle:
DC measuring voltage (with inverter stage)
Principle – Superimposition of DC voltage – Superimposition of an Adaptive Measuring Pulse
Application – Pure AC systems
Features – For small leakage capacitance. DC insulation faults can lead to false measuring results
AMP measuring principle (BENDER Patent)
Principle – Superimposition of an Adaptive Measuring Pulse
Application – Universal for all IT systems (AC, AC/DC, DC) in particular for systems containing converter drives
Features – For systems with high system leakage capacitance automatic adaptation to the prevailing system conditions
Operation :
The insulation monitoring device is connected between the phase conductors and earth.Continuously monitors the insulation resistance between the IT system and earth (PE).The measuring voltage UG generated by G is superimposed onto the system via the coupling R,a low pass and the measuring resistance Rm.An insulation fault RF closes the measuring circuit.The measuring current Im is flowing. Im causes a voltage drop Um proportional to the insulation fault RF at the measuring resistance Rm.Optically signals a drop in insulation resistance below a minimum value.Must comply with the requirements of the product standard for insulation monitoring devices IEC 61557-8.
Benefits:
Extensive personnel protection
High system safety and property protection
Preventive fire protection
High protection of the environment
Prevention of malfunctions
Less time and man power for fault location
Prevention of unnecessary repair work
Plannable down-times
Extended maintenance-free periods