I'm trying to simulate a circuit for a design I'm making. In a few parts of what I'm interfacing with uses relays that when closed will short a few power supplies to the chassis ground and in LTSpice and Circuit lab there is no "chassis ground" or "earth ground" just one ground. It doesn't short the rest of the circuits with power supplies tied to their own grounds just a few spots these relays are used. I've been trying to find the answer on here and elsewhere and found Need help modelling the circuit with parasitic capacitance of a power supply in this scenario in SPICE where he just hand drew it but is there a way to make it with subskt or is there a proper way to simulate this that won't skew my sim in weird ways?
Due to the nature of the SPICE (ever since 40+ years), the ground (
GND) node is global, but just a net, like all nets. And all nets can be renamed to whatever name/label/etc is needed. In this case, that chassis you're looking for, in whichever SPICE software, is just a cosmetic appearance for a net/node, labeled
earth, or whatever. In LTspice there is only one cosmetic like that, called
COM, but which has the same functionality of a common node (like
earth, etc, unless mentioned otherwise). It, too, needs to be referenced to ground, so some connection is needed, if you so choose to use it. This is true for whichever SPICE program that uses those nodes as floating nodes. Here's a minor example of usage:
R2 that connects the triangular shape ground (the ground,
GND) to the cosmetic
COM. Its value is
1g to simulate a separate, floating ground, but really it's there with the sole purpose of providing a reference to ground without which the solver cannot calculate. Also, the plot shows
V(2,COM), which is just a local reference for probing; can be anything, really.