For a constant current circuit, since the output current already is determined, how is the limited energy circuit determined?
More specifically you asked: It is understood how to calculate the maximum current to determine the limited energy circuit for a constant voltage circuit (tables 17 and 18). However, in the case of a constant current circuit, since the output current already determined, in this case, how to determine the limited energy circuit?
In response, the reference to Tables 17 and 18 in your inquiry is not clear. We assume you meant Figures 17 and 18 of IEC 62368-1 since Tables 17 and 18 are for determining the required clearances/creepage distances. We also do not quite understand what you meant by constant voltage although we suspect you probably meant DC voltage. It is also not clear if you were referring to an energy source in the context of electrical energy source (ES1, ES2, ES3) as defined in Clause 5, or in the context of power energy source (PS1, PS2, PS3) as defined in Clause 6. Please note, there is no definition for limited energy in the standard. Therefore, we can only provide a very limited response based on those assumptions.
For the ES levels, per Clause 5, the standard allows either measuring the voltage or the current as shown in Table 4, which shows the value in rms, DC and peak.
For the PS levels, the test methods are in Clause 6 using the circuit setups in Figures 34 and 35. This is used to determine the maximum output power under power measurement for worst-case fault and for worst-case power source fault. There are a few great answers on power level testing for IEC 62368-1 you may want to review by searching for "PS" under the topic section of https://62368-ul-solutions.com/engineers/ .
Given the unknowns in your inquiry, you are encouraged to contact UL for an in-depth consultation, either via your local account executive or via https://62368-ul-solutions.com/contact-ul.html .