You have a legal duty as an employer to ensure your employees are competent to undertake work. I'd like to think there was a simple route to show a person was competent to use a multi meter or isolate a fused spur without having to be a fully qualified spark.
This 3-day course might be what you are looking for:
Electricity for Plumbers | Learning for Life | BPEC
The BPEC Electricity for Plumbers course aims to help plumbers and heating engineers who work on central heating electrical control systems to work safely.
bpec.org.uk