VMB1RYS - power and type clarification?

What’s the maximum power that can be connected to VMB1RYS? The datasheet
Resistive load (water heater, towel heater). I understand that relay max current 10A @ 230 VAC with some safety margin gives 2kW. What puzzles me is “relay max. power 250 VAC”. I read VAC as Volt of AC, so it’s not really power unit. If VA is for apparent power than, what’s is C?

It’s probably a dumb question about typo on a datasheet, but I’d rather not see any smoke coming out of relay controlling the water heater/towel heater :slight_smile:

Also, I cant find any info if it’s solid state relay or a normal one? I’m thinking about using PWM (pulse width modulation) to regulate the power, but with normal relay that could produce very annoying sound or kill the relay if it’s switched too often.

I think VMB1RYNO might give me more safety margin with 16A / 230VAC



As a corporate decision about a year ago, all mechanical relays in current Velbus modules have the same specification.

Tolerate 16amp @ 230Vac (Volt @ Alternatively Current), with an inrush tolerance of 80amps. *

There was chat about testing a SSR, but as they work best with AC power, it would remove the option to control DC power and they tend to be <5amp

Please note, the shortest switching time with any Velbus relay is 1 second.

Does this help you?

VMB4RYLD has a PCB / module limit of 16amp for the entire unit

Whereas the VMB4RYNO and single relay modules are rated at 16amp.

That said, as a general point, I wouldn’t load any “smart” / control relay that much, I would always place a sacrificial Contactor in line with the load

My logic being, it’s quicker and cheaper to replace a £10 contactor than it is to replace a “user damaged” Velbus module.

The Velbus 10 year warranty is generous, but if a module has been overloaded, we won’t replace it.

1 Like

Perfect info, thank you! Considering the thermal inertia I can use PWM, measured in minutes.
I didn’t think about the sacrificial relay - that’s a brilliant idea! That made me thinking… for bigger loads velbus I/O + power relay might be a safer option.

The rating for a specific set of relay contacts to survive closure and opening is very different for AC and DC. AC goes through zero current 100 times per second, that helps stopping the current and therefore the arc that appears between the contacts when they open. For DC, that does not happen, the arc has to go out with distance between contacts and as long as the distance is not sufficient the arc persists and burns the contacts.

10A @ 230 VAC rated contacts will be completely destroyed by 10 Amps 24 Volts DC. So it is not a typo. It is just to remind you the 10 Amps rating only applies to AC current and the contacts can stand off far enough from each other to isolate 230 Volts.

Further, the RY modules do use mechanical relays. I would hesitate to use that in PWM operation. Also, the operation you are thinking about creates noticeable power peaks on the grid where a high frequency solid state PWM would not. With grid operators cracking down on peaks because of the EV charging that is going on, your option may also be less future proof.

1 Like

Thanks for the detailed reply.

Where did you get this information from?

All relay modules in the Velbus range use electromechanical relays.

There was talk of trying SSR, but nothing came of it.

For further technical support on advanced matters like this, may I ask you to create a ticket at -

1 Like

@pdhoogh Thank you for the detailed explanation, but my original question still stands.
I’m asking about the relay max power (yellow highlight):

@MDAR Perfect, thank you! I’ll ask the support and post back the reply.

Oh I see

Technically speaking, there are no components in the PCB path between the terminals and the relay contacts.

I think we say the default answer of 300Vac

But I also say as a default, “If you are planning on large loads, please use a contactor”

If only because these are cheaper and easier to replace.

@MDAR I know and respect you and your knowledge, I’m afraid I do not understand your question. With “mechanical relays” I do mean electromechanical and I don’t know the difference but that may be ignorance on my part.

The “Solid State Relay” - I think that is what you may mean by SSR, right? - have one downside in small modules: heat dissipation. Possibly the new silicon carbide tech may change that in the future.

@PrzemoF: 250 VAC for “power” does not seem to make a lot of sense indeed. Both the number and the unit of measure are off. On the website I couldn’t find what you copied here.

My apologies, I miss-read your reply.

We are talking about the same things

It doesn’t seem to be on the website, but I have a habit from my workplace to collect data sheets :face_with_monocle:. See the link in the opening post, right column, third row from the bottom. I asked the customer support - we’ll see what comes back.

The Customer Support replied with:

The 250 VAC max. power indication means,
That the relay contacts can handle a maximum mains voltage of 250 VAC.

Best Regards,
Velbus SL2 - Tech Service"