contactor surge suppression

[vjekobalas]

The thermostat on my water boiler has blown, so I cobbled together
a 230V circuit which switches on/off the 400V 3 phase contactor which powers
the water heater element. The 230V circuit consists of a 230V
electronic timer ,two 230V bi-metal temperature sensors in series (fail-safe for overheating
of boiler - they are attached to the water boiler) and an automatic fuse.
In order to avoid killing the heating element from overheating etc., I have the timer
switching power to it for a short time each hour during the night.

I was wondering whether I need to add some sort of surge suppression on the
contactor ?

[Alti]

In order to avoid killing the heating element from overheating etc., I have the timer
switching power to it for a short time each hour during the night.

So you switch power for a short time to prevent overheating a heating element. Details please.

[vjekobalas]

I am switching power to the element for a part of an hour eg. 20min but that is not
what I'm wondering about - it is whether the contactor  needs surge suppression
to protect it and the circuit that is driving it.

Forgot to mention the parts used:
230V driving circuit : 230V fuse (ordinary automatic fuse/circuit breaker) ETIMAT6 B10
https://www.tme.eu/en/details/002111514/circuit-breakers/eti-polam/etimat-6-1p-b10/?f45b=f45b

400V contactor:Z-SCH230/25-40
https://www.electricautomationnetwork.com/en/eaton-moeller/installation-contactor-230vac-50hz-4n-o-25a-2hp-eaton-moeller-248847-z-sch230-25-40

 The answer to the question is of interest for the life of the present components and as
I'd like to swap out the 230V electronic timer I'm using to switch the power to the 230V circuit
with a microcontroller driving a relay to add more intelligence to the setup.

[SeanB]

Not needed, a resistive heater load is not going to cause excess contact wear, and the contacts are rated for enough operations that you only will need to worry about this in 30 years, with one operation per hour. Contactor will wear out the core frame before then.

[vjekobalas]

OK looks like I need to be more specific - firstly, I was wondering whether the resistive heater
(three elements, and the heater is rated at 3kW) induced some voltage (I have no idea what
the wiring looks like inside the element) and because of this and initial switching , surge protection
was necessary.

Secondly, the heater element died once already and I was told that the heater is made to work in
combination with another source of heating (I have solar) i.e. the heating element is not made to heat
continuously - so I am switching power to it mainly during the night some 15min-20min
each hour, so power is switched anywhere between 10 - 15 times per day.

[Alti]

Secondly, the heater element died once already and I was told that the heater is made to work in
combination with another source of heating (I have solar) i.e. the heating element is not made to heat continuously - so I am switching power to it mainly during the night some 15min-20min
each hour, so power is switched anywhere between 10 - 15 times per day.

Someone was pulling your leg. You can run water heating element 24/7 if you like.

As for surge protection, this is provided externally at supply incoming point. Resistive heater does not induce any surges on switching.

[floobydust]

Heating elements are not purely resistive loads! EE's need to stop parroting theory. You can observe the arcing when a contactor shuts off.
Due to their coiled windings inside steel, along with their wiring, they have inductance. It does cause contact arcing and bursts of EMI. Contactors are designed to take arcing and it's no big deal for them. At the high currents multi kW it can be a problem smaller appliance relays end up having a shorter lifetime. For this reason, I always add a RC snubber across relay contacts, but OP's heater doesn't strictly need any. Safety standards include an endurance test with typically a 100,000 cycle requirement, to flush out bad designs with heater relay getting stuck on.
The EMI burst from contact arcing when opening can cause your nearby MCU or RPi etc. to crash and reboot, which is another reason for using snubbers.

[vjekobalas]

Much appreciated for the info ! - I was sure there was inductance involved and it should have some impact but
I have zero experience. Do you have a pointer on design of the snubber/component/type or is there some
standard component choice/components/type ? (Let's assume the future setup
with 230V circuit :  uC ->relay ->  automatic circuit breaker/fuse -> bimetal temp sensors => 400V contactor -> 3kW heater load)

[Terry Bites]

AC contacts are pretty reliable with reistive loads.
A 3kW heater won't have much inductance. A small RC snubber, say 100R in series with 100n across the contacts will help polish off any transients.
It redcuces RFI. I had a mate whos C64 (a long time ago) crashed when ever the heating came on- an RC sbubber on the thermostat stopped it happening again.

Put a MOV across the contacts as well for good measure. A low cost MOV-07D471K is good enough.

[vjekobalas]

Can you please be a bit more specific about snubber resistor and capacitor specs./type ?
Do you maybe have a pointer to some reference on calculating these values ?

[floobydust]

Snubbers have a lot of math for their values, too much because you don't always know all the inductance values.
In a perfect world one would use a scope for tuning the values, but here that's a bit dangerous with the high mains voltages.

Another way, it's easiest to use common, standard values like 0.1uF and 100 ohms 1/2W. I would use 47-100nF X-cap and a flameproof/fusible 1/2W resistor because there is no fuse if the cap fails shorted. A heater has much less inductance than a motor so a smaller than usual capacitor is fine. Quencharc are convenient, integrated but expensive, the datasheet has some advice.

You have 3-phase power 230/400 so 400VAC phase-phase? Each contact would see 400VAC? Look at Epcos/TDK B32912A5473 or Kemet R47 series. I would use 47nF 440VAC X capacitors and 33-100R 1/2W. You will also need a smaller snubber (voltage) across the small relay's contact to stop it from arcing due to the main contactor's coil inductance.

[MrAl]

That's interesting about the heating elements not being purely resistive.  If you take a look at old style soldering irons you can note that they say 120vac 60Hz or something similar and that is a specification for an AC voltage not DC.
Many of the newer ones like you find with soldering stations can be DC but that's because they actually are resistive.  So it depends on the actual model.
If you see a relatively large voltage surge from the device after the power is abruptly disconnected, then there must be some inductance in there or else you have long wiring that has enough inductance to cause that surge.  You can also look at the specs for the element if you can find them, and if it says 120vac or 220vac or 440vac or anything with just "AC" then you know you have some inductance in there.  If it was non inductive it would most likely say "AC or DC".
Maybe you can look into that to see if you can find the specifications for that particular element(s).  If not anything else, it would be interesting to find out.

[Vovk_Z]

Powerful heaters usually don't have large inductance, but even a small one together with a large load current still makes high voltage and so an arc.
V = -L × dI/dt. 

[jonpaul]

Snubbers

https://en.wikipedia.org/wiki/Snubber
https://www.homemade-circuits.com/prevent-relay-arcing-using-rc-snubber-circuits/

Contactors have a high L coil with a high voltage kick when turned off.

Contact snubbers are commonly a 10..100 Ohm carbon film/carbon comp R 1..2W in series with a 1..50 nF cap rated for the safety and voltages involved.

Plastic film or HV discs and VED X2 rated are common.

Jon

[Seekonk]

Be careful, snubbers create another failure point which could cause a heater to stay on. There is a new UL approved solid state device about the size of a microswitch (DC and AC versions) which evidently provides a momentary short till the contacts open far enough. Video looked pretty interesting.  I just saw it the other day and thought it was bookmarked.  Inductance just increases the voltage, so the gap has to get wider. Anytime I see the word SOLAR I know there is a bunch of voodoo coming. I heat with solar using pulses and I have no problems with ringing. An immersion heater that can't take continuous operation, I don't think so. If it did, it wouldn't be a 15 minute time period. 

Found it Has to be quite expensive, most interesting thing I have seen in a while they have a nice video.
https://arcsuppressiontechnologies.com/wp-content/uploads/2020/09/NOsparc-MGXDC1F-User-Manual-104-0045C.pdf

[jonpaul]

https://arcsuppressiontechnologies.com

$45..67 or higher for a MOV Metal oxide Varistor worth perhoas $0.75..$5.

Great marketing.

In genenral all MOV are Limited life, susceptible to fire and catastrophic failure if overloaded.

Just my experience with MOV varistors


Jon

PS: (invented at GE in ~ 1900 by Dr Charles Proteus STEINMETZ)

[Seekonk]

THAT IS NOT A MOV. There are specific warnings not to place across coils or any load.

I wonder with the OP issues if he isn't putting substantial DC voltage from solar panels and that is his arcing.  On youtube they show putting several thousand uF electrolytic on contacts to prevent arcing. What could go wrong with that? 

[BILLPOD]

Sounds like the perfect application for an SSR, (solid state relay),
no contacts to arc.

[vjekobalas]

The solar part is solar water collectors/no electricity