Failsafe fuse blowing with SCR - Page 1
Forum section: electronics / projects

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 13, 2021, 03:16:59 pm
content: Hi folks.I'm looking for failsafe measures for the water heater. It is regulated by SSR and I need protection when it might fail closed.And I wonder if I can "safely" blow fast fuses with an SCR crowbar. Without much disruption to other circuits.And what fuse I2t rating compared to SCR shall be to blow them reliably. As superfast fuses tend to be expensive and are also somewhat scarce now.Another considered solution is a circuit breaker with a dedicated trip coil.

author: bdunham7
author_rank: Super Contributor
timestamp: September 13, 2021, 03:59:27 pm
content: The first thing that comes to mind is a latching contactor, but those are expen$ive. A regular contactor would work, but those aren't super cheap and would require the coil to be energized all the time. I'm not a fan of the crowbar/fuse method, especially on HV or mains circuits where the fault energy is pretty high and there is a risk of blowing stuff up. Depending on the size/power of the water heater, perhaps a simple mechanical thermal cutoff switch would work?

author: Zero999
author_rank: Super Contributor
timestamp: September 13, 2021, 04:41:23 pm
content: Presumably this is to protect it against overheating?How much current does the heater use?If it's under 15A, then thermal fuses which can handle that current, are widely available.If it's a much higher current, put a contactor in series with the element and use the thermal fuse to switch the coil.

author: wraper
author_rank: Unknown
timestamp: September 13, 2021, 04:56:01 pm
content: Another option is adding a relay which turns on power-on. But SSR used as an actual switch. If there is an overheating, you turn off the relay. Or activate it if it has normally closed contacts as already suggested.

author: langwadt
author_rank: Super Contributor
timestamp: September 13, 2021, 05:03:25 pm
content: Quote from: Zero999 on September 13, 2021, 04:41:23 pmI've used an RCD as a latching off, a small current around it triggers it just like the test buttonhttps://en.wikipedia.org/wiki/Residual-current_device#/media/File:FISkizze.svg

author: wraper
author_rank: Unknown
timestamp: September 13, 2021, 05:06:14 pm
content: Also blowing mains fuses is not a good idea to begin with. Quite likely you will also activate circuit breakers upstream, and unintentionally leave other things without power. Which may turn out into a big clusterfuck, say fridge left without power while you were away for a week.

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 13, 2021, 05:42:13 pm
content: Fast fuse gR should not trip circuit breaker even same nominal current, it has significantly lower I2tUsing RCD is an interesting option/alternative to circuit breaker with trip coilRelay is the cheapest optionIt is for 6/12kW unit

author: wraper
author_rank: Unknown
timestamp: September 13, 2021, 06:02:35 pm
content: Quote from: Miyuki on September 13, 2021, 05:42:13 pmAs if, I've seen plenty of times when something like PSU with 4A fuse when failing also trips 16A circuit breaker.

author: bdunham7
author_rank: Super Contributor
timestamp: September 13, 2021, 06:35:26 pm
content: Quote from: Miyuki on September 13, 2021, 05:42:13 pmI don't know about in your locale, but the breakers in use here have two curves and I2t is not really a valid concept. Even if you look at a datasheet you may be a bit mislead. They have a normal thermal action and then a magnetic trip action. The magnetic trip action is very, very fast to activate, but then it takes some milliseconds for the mechanism to actually complete the disconnection--but usually rated at one-half power cycle. The data sheet will indicate the clearing time--that is how long the breaker takes to open. However, the time it takes to irreversibly set the mechanism in motion is much, much shorter at high fault currents--typically 8-10X the rated current of the breaker. I just did this the other day testing a pump motor, which was unfortunately miswired by persons unknown. A 7.5 slo-blow motor fuse blew, but the 20A breaker tripped off as well.

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 13, 2021, 07:58:56 pm
content: Quote from: bdunham7 on September 13, 2021, 06:35:26 pmYou are right, my bad. Common breaker will trip even after short pulse, just with some delay (what cause high I2t value)

author: NiHaoMike
author_rank: Super Contributor
timestamp: September 14, 2021, 04:34:31 am
content: Cheapest, most reliable protection against thermal runaway is to simply add a thermal fuse.

author: Ground_Loop
author_rank: Frequent Contributor
timestamp: September 14, 2021, 10:52:58 am
content: I hate to be that guy that tries to steer the OP reasoning in a 'better' direction by asking a bunch of questions, but I'm curious nonetheless. I've been working with electric water heaters for decades including making significant modifications for industrial and scientific applications. Water heater control is a mature technology and also regulated. I've never seen a runaway control in a water heater. What has you concerned?

author: Miyuki
author_rank: Frequent Contributor
timestamp: September 14, 2021, 01:00:40 pm
content: Quote from: Ground_Loop on September 14, 2021, 10:52:58 amI was basically just thinking about the possibility of avoiding a big contactor (energized thru failsafe thermostat) in the circuitI'm concerned with two failure modes:1) SSR fail short2) controller go crazy

author: TheMG
author_rank: Frequent Contributor
timestamp: September 14, 2021, 07:41:11 pm
content: All standards-compliant water heaters on the market will already have some form of overtemp protection.If you're that worried about it and don't want to use a contactor you could get a shunt-trip circuit breaker. Just like a normal circuit breaker but it has an extra solenoid in it that when energized will trip the breaker.