I want to make a electric leakage protection circuit for a water heater in case of heater gets corrupted in time. We use rezistance 220V - 1500 watt. We grounded the water (because rezistance have coating outside and doesnt convey electric to water) but when there is electricity in water, RCD at office cuts off electricity for all devices. We only want heater to turn off. So, we bought another RCD and connected it to water heater, then we plug water heater to electiricty. It works but two RCD triggered at the same time. We only want one of the RCDs that connected to the heater to trigger. I can't connect different RCD in fuse board because system must be portable and this safety system must be included to main circuit.

Is it possible to do this with RCDs? Or can we add something after RCD of water heater to delay the current, so RCD of office wont trigger?


2 Answers 2


If you can incorporate an isolation transformer into your build then this will prevent the main fuse board RCD tripping. The isolation transformer fits between your local RCD and the incoming AC power feed from the fuse board.

Alternatively, if you can get an RCD that can be easily dismantled and modified, you could make it more sensitive by looping the through going live and neutral wires through the internal magnetic toroid 2 or 3 times.


There are a few problems here.

  1. Why are you using a faulty heater?
  2. You should be earthing the heater metalwork, not the water.
  3. You need to install the RCD on a separate circuit from the main fuseboard.

Update after OP's update.


simulate this circuit – Schematic created using CircuitLab

Figure 1. Test circuit.

The standard technique for this is to add an isolation transformer. When one side of the transformer secondary is earthed (grounded) it replicates a mains supply with one conductor neutralised. Now any leakage on the test load will return to the transformer N1 terminal via the ground connection. The RCD on the primary (the office RCD) will not sense a current difference between L and N so it will not trip.

The transformer will have to be rated for maximum load, so 1.5 kVA.

The other test you should be considering doing is an insulation breakdown test using a "Meggar" or simular 500 V to 1000 V tester.

  • 1
    \$\begingroup\$ 1- Not planning on using faulty heater but I am testing with one in case of coating corruption of heater in time. 2- Outside of heater is coated for protection and doesnt convey the electricity(in normal condutions), so I decided to earth water. Is is wrong thing to do? Also I use resistance as heater, and it doesnt have grounding inside. 3- I would do this but my boss wants something portable. He doesnt want to change anything inside fuseboard. \$\endgroup\$ Commented Jan 25, 2020 at 9:31
  • \$\begingroup\$ All that information should be in your question along with the number of phases of the heater and the maximum power rating you need to test. Hit the edit link below your question ... \$\endgroup\$
    – Transistor
    Commented Jan 25, 2020 at 9:54
  • \$\begingroup\$ Okay I updated question. \$\endgroup\$ Commented Jan 25, 2020 at 10:12

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