Delayed Turn on Relay

I am building a new power supply in my motorhome, various pieces of equipment run from the leisure battery, I have fitted a 240 volt to 13.5 volt power supply to run things when hooked up to the mains, i need to install a changeover relay to feed everything from this rather than the LB's. when connected to the mains.the power supply 'ramps up' to13.5 volts, I want the relay to delay turning on momentarily to allow the voltage to stabilise before changing. is there a simple way to do this?

Use a Directed 528t variable timer relay from Car Audio and Security Hayes Middx.
Frankly I cant see how 13.8 volts will damage anything, most modern alternators put out about 14.5 and modern car electrics, light bulbs etc. are rated at 15 VDC.

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

https://www.the12volt.com/installbay/forum_posts.asp?tid=140844#715440

If you prefer something already made, there are 12 volt time delay relays available. I've seen some from Hella (haven't tried them), and have used Altec Automotive. Some are fixed, some are adjustable.

*It's also important to note, although the pin configuration is the same as standard automotive relays, many of these are only rated for 10 amps or so. If you will need more current than this, you can use the time delay relay to trigger a standard automotive relay.*

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My ex once told me I have a perfect face for radio.

I am not worried about the 13.5 volts damaging anything! I'll give you a scenario, it's a cold evening when i pull onto a site, the night heater is running, pulling around 5 amps, the fridge is running pulling another 3 to 4 amps, someone's in the back making a brew so the interior lights are on 1 to 2 amps, maybe filling the kettle so the water pump is firing up drawing another 4 amps, the instant i plug the mains cable in the 240 volt power supply is faced with a demand of 15 amps, that may be fine, i just thought a slight delay to allow the unit to fully power up before any demands were put on it would be wise!

That lot is a lot less than starting a car with the headlights and maybe hot rear window on, see where I'm going with this?
Don't worry, you don't need it!

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Don't think you understand where i'm coming from with this, starting a car with headlights on is drawing power from a battery which can supply hundreds of amps instantly, the instant i plug 'shore power' into the van the changeover relay will demand current from the switched mode power supply,i just want the power supply to turn on a couple of seconds before any demands are made of it!

So back to my original answer

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Amateurs assume, don't test and have problems; pros test first. I am not a free install service.
Read the installation manual, do a search here or online for your vehicle wiring before posting.

Ok, just thought it would be possible to get a delay with a cap and a couple of resistors!