# Simulate Earthing in LTSpice

I wan to simulate earthing or frame ground which is separate from circuit GND in LTSpice. I want to simulate line filter using Y capacitors C2 and C3 in the below image.

See below image for clarity.

I can consider negative terminal of input source as Neutral, so will that be ground? If yes then what should I connect to common connection of C2 and C3 where GND is connected right now?

There is one option which is to put two different ground in the simulation, one for Neutral and another one for frame ground. Now, what will be the parasitic elements to be put between Neutral and frame ground to simulate frame ground as earthing?

• In SPICE, the ground is the general reference. LTspice allows you a second ground symbol, but that's just a convenient way to differentiate between that another node. Under the hood it's just another node (just as the ground is, actually). How you use that special, or any other named net, is up to you. Usually, it's a parallel RC, with Meg or G as a value, and a few pF(more or less) worth of capacitance. Of course, this implies air contact. Real ground has to have a more complicated impedance (which I don't know). – a concerned citizen Nov 16 '17 at 8:19
• I have posted an answer, but if you could provide a bit more details about your application, the answer could be tailored for your particular real life application. – Jurkstas Oct 19 '18 at 22:10

1) It dipends on what noise you want to simulate, common mode noise or differential mode noise. For the differential mode you can avoid to connect the middle point between the capacitor.

2) I don't think that you can put different ground on spice

• I am mostly concentrating on the common mode noise. If I don't connect the middle point between capacitor, wouldn't that change the return path for the noise? – Vishal P Nov 15 '17 at 11:21
• Ok, then i think you will put another voltage generator between neutral and ground. To better understand it would be better to see the complete circuit. – RodezIO Nov 15 '17 at 12:57
• Thank you Rodezio, but what should be the parameters of voltage generator? Should it be sine wave generator or pulses or DC? That's what is missing what should we put between Neutral and earth to simulate actual scenario? – Vishal P Nov 15 '17 at 13:43
• Indeed it depends on what is the noise signal that you want to simulate. It could be a surge, a burst or any other kind of undesired signal. What noise do you want to simulate ? – RodezIO Nov 16 '17 at 7:46

You can simulate anything you can model. There is only one ground net, but it is only the reference point for simulation - the zero V net. If you are modeling circuit ground and frame ground, it means that you have two nets. How are those nets coupled? Common mode usually enters through parasitic capacitance. So add a "Frame" net and add some capacitance to both wires of your power supply. For example like this:

In this circuit it is assumed that:

1. Y filter common point and neutral are connected to PE, no parasitic resistance nor inductance.
2. Frame is only coupled to the circuit via parasitic capacitance, 10pF to both neutral and live.

You can observe that the frame is essentially floating between the two potentials. If you would add some details on how your circuit is connected, the answer could be more detailed.

• Can you also model to inject interference via capacitive and inductive coupling? – atmnt Nov 22 '18 at 17:37
• Yes, you can model and then simulate both inductive and capacitive coupling. Modeling is the hard part, though. – Jurkstas Dec 10 '18 at 18:28

Do a search for "earthing systems." If you have a neutral and PE running to this circuit, then both are connected together back at a panel. If your frame is connected to PE, which would be required for a UL listing, frame is connected to ground. In this case, the - lead of V2 is ground and the junction of your filter is connected to ground too with perhaps a very low series resistance and some inductance.

Probably a better idea is to model a LISN (line impedance stabilization network). This is what would be put between a power source and your device when it is tested by a certification lab. V2 would then be your ideal grounded power source and the line, neutral and PE would be run through your LISN circuit to your device circuit.