christelle avatar image
christelle asked

Multiplus Ground Relay with RCD on output AC

I have a 1600VA Multiplus on a vehicle with a 20ma RCD on the AC output of my Multiplus.

With this configuration, I don't understand the point of using a ground relay when the AC input is disconnected. To explain my point of view, I hypothesize that there is a malfunction in my installation and that the phase L touches the bodywork of my vehicle.

Option 1: Ground Relay Off
During a malfunction of this type, the most problematic in this configuration without ground relay is that the bodywork of the vehicle could be connected to the AC phase of the Multiplus without causing a stoppage of the Multiplus. In this case, if a person touches the bodywork and he is inside the vehicle, he is then touching the phase but the current has no way of returning to neutral, so no problem. If a person touches the bodywork and they are outside the vehicle, then the current could pass through it and go directly to earth, thus causing an imbalance between phase and neutral and the RCD will then trip to protect the person.

Option 2: Ground Relay On
If the phase of the Multiplus connects to the bodywork, phase and neutral will be connected together causing the immediate tripping of the Multiplus with a short circuit. This option is more secure because it immediately causes a stoppage of the Multiplus.

On the other hand, there is another hypothesis where the option 2 can be a problem in my opinion. If a person touches the output phase of the Multiplus (example of an intervention on a AC power socket in the vehicle) and this person is also in contact with the bodywork of the vehicle, in this case there is a danger. Indeed, the current will pass from the phase in its body will arrive on the bodywork connected to the neutral and the RCD will not detect any imbalance. The person will then transform into an electrical charge without there being a tripping of the RCD or Multiplus.

For my opinion, the option 1, RCD and no ground relay protects in all scenarios while option 2, RCD and ground relay may cause security problems in the last scenario above.

Thanks for your help

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2 Answers
southsideph avatar image
southsideph answered ·

The RCD on the inverter/Multiplus AC out, measures imbalance between I in (Current) I out. These in normal operation should be equal, if not there is a fault. In a shore connected system testing the RCD taps the current flow to earth causing an imbalance, so tripping RCD which would be correct if there was a fault or a person in the circuit.

Off shore power there is no Earth (Ground) or no reference source so the RCD cannot test correctly and could be faulty - dangerous even. The earth relay connects the Earth to the Neutral when not on shore power so presenting a Earth/Ground reference so the RCD can be tested correctly.

The test button is a mechanical test of the RCD and not an actual fault test - so the RCD may trip when the test button is pressed but does not trip in a fault scenario thus may actually be dangerous.

Hope this helps a little

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Kevin Windrem avatar image
Kevin Windrem answered ·

Programming the Multi's internal ground relay to remain open when inverting is unsafe and usually not legal. There must ALWAYS be EXACTLY ONE connection between safety ground and neutral in any system even when isolated from the grid as is the case when the Multi opens it's AC input relay. The reason there is programming to keep this relay open is when there is another neutral/safety ground connection downstream of the Multi.

The purpose of bonding the neutral to the safety ground is so that an overcurrent device will trip if fault current flows an alternate path back to the service entrance panel where the neutral/safety ground is made. If you don't have a neutral/safety ground bond in the system (Multi inverting but internal ground relay open for example), a fault will cause the hot leg to approach the safety ground, pushing the neutral in the other direction. Essentially, the rules of hot and neutral reverse. This in itself won't cause unsafe conditions unless another device has a neutral/safety ground fault. If so, large currents will flow in all wiring including safety grounds between devices. If the hot/neutral fault occurs in a device with a small safety ground wire then it's possible no overcurrent devices will trip and the small safety ground wire might burn.

An overcurrent device will not trip in this situation. Small amount of current may flow between the neutral and safety ground (chassis). But I don't think you'd see any voltage on the chassis relative to earth UNLESS there is no connection between safety ground and earth where the system connects to the grid.

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