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I have a heater element that is controlled by a solid state relay from main power. I know it is customary to place a MOV in parallel when switching inductive loads to clamp the voltage to prevent damage when turning off the load. Is this completely unnecessary for resistive loads?

Besides that, I want to protect my circuit from voltage surges from the main so I will put a MOV anyways. I was wondering if I should get a larger MOV (more joules) to take into account the potential overvoltage from switching the heater element.

From the manufacturer, the SSR recommends putting a MOV in parallel. Considering the only inductance I have is from the copper wire which ends up being somewhere around 50 microhenries and using a zero crossing ssr, I feel like the overvoltage will be negigible but I am not sure. This will run on AC main power and 600A and my SSR is plenty overspecced. (rated to run at 1200AC but using 480AC).

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  • \$\begingroup\$ You may want to use multiple MOVs i.e. across L-N and across L-PE and N-PE, if your system has PE (protective earth) connection. \$\endgroup\$
    – Rohat Kılıç
    Commented Mar 10 at 11:57

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I know it is customary to place a MOV in parallel when switching inductive loads to clamp the voltage to prevent damage when turning off the load. Is this completely unnecessary for resistive loads?

You need the MOV because even though the load may be resistive, the wiring will have inductance that can cause a back-emf when the SSR deactivates. This applies to MOSFET based SSRs.

From the manufacturer, the SSR recommends putting a MOV in parallel.

I would follow what the manufacturer of the SSR recommends.

using a zero crossing ssr

If that means that the SSR switches off on the zero cross of the current (i.e. it is a triac-based SSR) then sure, that will cause very little back-emf and you probably don't need an MOV for that however, surge voltages from indirect lightning can easily cause 1500 volt peaks on the AC feed.

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  • \$\begingroup\$ I am unsure how to calculate the back-emf when the ssr deactivates. Yes I will properly spec a MOV for surge voltages from lightning but I was wondering if I need to over spec it to take into account the energy from switching off the ssr. The manufacturer recommends a mov and a r-c snubber when used for inductive loads but I cannot find any recommendation for resistive loads. \$\endgroup\$
    – realm god
    Commented Mar 10 at 12:21
  • \$\begingroup\$ @realmgod well, you have peak current at the point of deactivating the SSR and, you have copper inductance (mentioned in your question). The energy that is stored is \$\frac{1}{2}L\cdot I^2\$. It's probably not much (a few milli joules) given that you mentioned 50 uH but, it's for you to decide. \$\endgroup\$
    – Andy aka
    Commented Mar 10 at 12:33

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