Can current flow through a relay's NO channel when it's not energized?

Can current flow through a relay's NO channel when it's not energized?

I'm an electronics newbie and I built a circuit where a relay is operated by a Raspberry Pi Zero W. See circuit image. The relay used is a SRU-06VDC-SD-C. It's rated for 6v but I hear it click open and closed using the 5vdc from the Pi.

I would use this to turn my heating on and off at home, the circuit that switches heating on and off is a 110V DC line coming from my heating installation (my AC voltage is 220V). When this circuit is closed, the heating is turned on. This line is wired to the NO contact.

It worked fine for weeks when testing but when actually wired up to the heating it worked for about a week but now the heating is always on, even when I explicitely do not energize the relay. The heating is even turned on when the pi is switched off. When I measure with a multimeter, the meter claims the circuit is not closed. Could someone tell me what I'm doing wrong please?

• My guess is that it couldn't take the load and has failed, stuck closed. If you link to the datasheet we can have a look at its DC ratings. Nov 13, 2017 at 18:51
• datasheet.fr/datasheet/958849/SRU-06VDC-S-D-C-pdf.html Nov 13, 2017 at 19:02
• That link is to a web page - not a datasheet. It looks like a dodgy site so I'm not going to click anything on it. Nov 13, 2017 at 19:18
• Added image of relay Nov 13, 2017 at 19:33
• What is the inrush current of the heating system? and something sounds wrong about a 110V DC line.... Nov 13, 2017 at 19:47

simulate this circuit – Schematic created using CircuitLab

Figure 1. A single-pole, single-throw (SPST) relay.

• The relay is drawn in its unpowered or de-energised state.
• The common pole wile therefore be connected to the N.C. contact.
• On energisation of the coil the contact will change position and the common pole will then be connected to the N.O. contact.

Can current flow through a relay's NC channel when it's open (= relay unpowered)?

"Open" in the context of relays means that there is a gap between the contacts so current will not flow. However, you are trying to describe the relay unpowered in which case the N.C. contact will be closed and current will flow.

It worked fine for weeks when testing and when actually wired up to the heating it worked for about a week but now the heating is always on, even when I explicitely close the relay.

I think you mean "when I explicitly energise the relay".

If the heating is staying on then the contact is probably welded.

It's even on when the pi is switched off.

You should expect this if you used the N.C. contact.

When I measure with a multimeter, the meter claims it's closed.

If you measure the N.C. contact with the relay de-energised then this is the expected result.

Could someone tell me what I'm doing wrong please?

It could be one of a number of things:

• You have used an AC-rated relay on a DC supply.
• You have used the relay to switch a voltage higher than the contact ratings.
• You have provided no snubber across the load to protect the contacts.
• Thank you for pointing this out! I've updated my question to make it more clear... Nov 13, 2017 at 19:16