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Suppose a mains powered device has a PSU in the assembly that has a detachable cable to the mains. Is it just the PSU that needs to be UL approved/compliant? Is the whole device therefore UL-compliant as long as the PSU is?
Let's say an electric golf cart that takes 48VDC for charging the SLA batteries. Does the cart itself need to be approved or does only the charger need to be approved?
UL does not approve anything. They publish standards, maintain a list of products that they have tested and/or evaluated to determine that they meet those and other applicable standards, and authorize products that have been listed to be marked with the UL label. In the USA, they are one of several nationally recognized testing laboratories (NRTLs) that provide a similar service. Other NRTLs evaluate products to UL standards, list the products and authorize their own label. In the USA, "approved" means acceptable to the authority having jurisdiction.
There appears to be a standard for information technology equipment (ITE) that allows it to be powered by a listed power supply designed for such equipment without requiring the ITE to be listed.
Golf Carts
In the USA, the standard for golf carts seems to be ANSI Z130.1. There may be NRTLs that will certify conformance with that or manufacturers may assert conformance based on their own evaluation. The standard seems to apply to all aspects of electric motor and engine driven carts.
No, the whole needs to be compliant.
If device A is UL compliant and device B is UL compliant, it doesn't impy A+B UL compliant. It has to be tested/proved.
Moreover, if device A is UL compliant, but device B is not UL compliant, this doesn't imply A+B is not UL compliant.
The short answer as @Huisman said is yes, the cart will likely still need to be Listed (Listed is the correct term for approved, compliant etc. Not super important, but correct).
As you alluded to, there are exceptions based on the input power to the device. I don't know which standard golf carts are evaluated to, my experience is in Industrial Control Equipment. In ICE, for instance, devices which are powered by Class 2 power supples have most, if not all requirements waived:
From Omron:
Furthermore, a benefit of using the power supplies that have received the Class 2 approval is that the output has the same Class 2 safety level. When applying for safety standards approval for the equipment, in some cases it is not necessary to obtain safety standard approval for the connected device (load) when the device (load) is connected to the output of a power supply that has received Class 2 approval.
From CUI:
The limited output voltage and power delivery capabilities of Class 2 power supplies are recognized to be of lower risk to fire initiation and causing electrical shocks, which allows for lower cost wiring methods to be employed.
In your case, the charger and battery will very likely require testing. Depending on what testing the battery has already undergone, you may be able to waive some or all testing "downstream" i.e. the rest of the cart.
