Floating DC System & criteria in choosing the right DC Earth Fault Locator

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The unearthed systems or floating systems are being used in places where supply interruption is not acceptable, and supply interruption can cause a fire hazard or health hazard in the event of a fault. The most common use case for floating systems is at a power plant. Here the unearthed DC system is used as a protection system. The protection of the high voltage system is at risk if there is failure in the DC system. The other applications are: Operation Theatres (AC/DC), Ships (DC), Manufacturing Plants (AC/DC), Process Plants (Unearthed AC/DC System), Data Centres (DC), Control Rooms (DC), Airports (AC/DC), Railways (DC/AC). In addition, tunnels and below ground areas, the lights and ventilation system must work faultlessly; submarine crews as well as astronauts in space equally depend on a reliable power supply.  

Advantages of FLOATING / Unearthed Systems:

High reliability: Even if any fault occurs, the system continues to run smoothly. Also, in the event of an insulation fault – even if there is a dead earth fault – shutdown is not required. In an unearthed system, two insulation faults occurring at two places do not blow the fuses, but there is a definite risk of fire, but it is very low.

Online/Offline monitoring: The system can be monitored for faults even during operation or in offline mode, and faults can easily be monitored, unlike earthed systems.

Disadvantages of FLOATING / un earthed systems:

Automatic fault isolation is not possible as high current will not flow in the fault part. It requires a dedicated fault monitoring unit to detect faults as and when the first fault happens. The system becomes too complex because of leakage capacitance.  

Solution: For an unearthed system to protect people and installations against the hazards of electrical current by detecting and signalling them at an early stage. Unearthed systems with earth fault monitoring represent the ideal technical solution in these cases. For this reason, unearthed systems are used in particularly critical applications like control circuits, medical locations, or mobile power generators, and they are also found in newer applications, such as electric vehicles, photovoltaic systems, or industrial plants with variable-speed drives.

Therefore, choosing the right tool to detect the fault is highly important. The guide below highlights key features one should look for while selecting the kit.

Selecting the Right DC Earth Fault Locator:

  • It is advised to choose a kit that provides higher output DC impedance (greater than 100 kΩ). This will avoid creating a faulty alert on the DC system, also safe for sensitive relays in the DC system. The possibility of maloperation of the DC system is reduced by choosing this method. Very often, the kits available on the market will ground the DC system on one pole with low impedance (1/2 kΩ a bit higher) and switch between one pole and the other with very low impedance. This is like creating almost a dead fault and the battery system will feed this fault.
  • The DC system is typically 240V DC, with no more than 10% AC ripple on the system. Hence, a kit injecting less than 24 V AC and a frequency other than 50Hz is preferred. Injecting higher voltage can have an effect on your DC system. The kit injecting a frequency other than 50Hz is preferred in order to avoid interference due to 50Hz pickup. Be sure the device matches this condition.
  • The DC system has sensitive relays and they can mal-operate or trip if the transmitter injects more current. Hence, the transmitter should inject very less current (around max. 2mA short circuit current or less) so that the sensitive DC system relays do not mal-operate or trip at any condition. A kit injecting more current is not ideal for the DC system. The receiver should be made sensitive enough to pick the small current injected by the transmitter and filter all the noise.
  • The DC feeders/cables are capacitive, and the kit should not erroneously detect the capacitance effect of the cable as a fault. The kit should automatically balance the capacitance without reducing sensitivity of the receiver so the operator is not worried about balancing the system capacitance. For example, the kit balancing capacitance should not reduce the sensitivity of the receiver. The kit having a sensitivity of 100 uA should have the same sensitivity even after capacitance balancing, and it should not go to 1mA after capacitance balancing.
  • Choosing the kit with its own source helps in identifying the fault even in offline mode.
  • A receiver that detects even the smallest resistive current without picking up on cable capacitance or increasing the voltage on the transmitter is considered ideal for use. For example, the transmitter injecting 15 V and receiver picking 400 kilo ohms (15V/400 k ohm) can pick < 50 uA where-as the kit injecting 1000V and picking 1 Megaohm is only picking 1mA (1000V/1Mohm). The former is better than the latter, at least by 25 times.
  • The kit should be as lightweight as possible. The lower the weight, the better it is for the user to carry for testing.
  • LCD displays are preferred in modern eras, and the DC system voltage should be readable even when the transmitter is injecting signals into the DC system without the use of a multimeter and other accessories.
  • The kit should not disturb the DC system while connecting or during operation.
  • The power supply of the kit must have rechargeable or replaceable batteries that are easily available in the market.
  • Usually, some feeders are big and some are small. Hence, a kit giving two current probes with big and small sizes (both with high sensitivity) is preferred. A kit having the same sensitivity for both probes and high sensitivity (less than 50 mΩ) to detect very small faults is preferred.

Now you are better equipped with the right information to choose the right kit and keep the DC system active and ensure smooth operation. Still confused about finding the right kit? Worry not, we have the right kit that matches all these criteria and a lot more features. Our DC ground locator,  MGFL, helps in locating, tracing and pinpointing DC ground faults in any floating DC system of up to 300V DC in online and offline conditions.

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