Switching electric shower ON/OFF causes mains transients? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: August 07, 2021, 02:28:32 pm
content: Hi,I had three different microwave ovens blow up in a year....in all cases i took them apart, and found they had no MOV at the input...just a little X2 capacitor, and the input fuse had crowbar'd in each case. They all worked again when fuse replaced.In each case, when they blew up, the microwave was being used at the same time as the electric shower.....and the shower (10kW) was being switched on and off because it was being used to wash and rinse clothes in bowls. Do you believe that the electric shower being switched ON and OFF could have generated the mains transients that blew up the microwaves.?How powerful do you think this type of transient is compared to typical mains borne transients.?

author: Gyro
author_rank: Super Contributor
timestamp: August 08, 2021, 10:33:52 am
content: If you're worried about transients then have an 18th Edition SPD fitted at the consumer unit, then you'll know.P.S. Maybe your solar inverter doesn't like the load dump, hard to keep track with so many threads.

author: f4eru
author_rank: Super Contributor
timestamp: August 09, 2021, 10:40:51 pm
content: It could very well be the "self healing" of this capacitor in action when those voltage spikes appear and trigger a faiure point in the cap.Use MOVs.https://www.researchgate.net/figure/Self-healing-mechanism-in-polymer-dielectric-capacitor-79_fig14_283711298

author: Circlotron
author_rank: Super Contributor
timestamp: August 10, 2021, 02:45:06 am
content: Does your water heater use relay contacts to turn the AC power on and off? If instead you use a triac or a pair of SCRs they will turn off at near zero current, minimising the voltage of any spike caused by the stray inductance of your house wiring. You are switching 40 amps here. Arranging for zero voltage turn-on is a good idea too. Won't help your problem but is good practice.

author: jaycee
author_rank: Regular Contributor
timestamp: August 10, 2021, 07:24:47 am
content: As the electric shower will be on its own 10mm2 cable, I can't see it causing any major transients that would get onto the circuit that feeds your kitchen. Perhaps the microwaves are just cheaply made crap!

author: Gyro
author_rank: Super Contributor
timestamp: August 10, 2021, 01:16:52 pm
content: Quote from: Circlotron on August 10, 2021, 02:45:06 amYou can't just go messing with the electrics / electronics inside an electric shower, these things have safety standards and approvals! I remember treez (Faringdon) creating a thread about it once before and it's a complete non-started from the safety and insurance perspective.

author: David Hess
author_rank: Super Contributor
timestamp: August 10, 2021, 07:32:52 pm
content: Quote from: Circlotron on August 10, 2021, 02:45:06 amThat is my guess. If you have a DSO, then it could be used to monitor the line voltage for spikes to get a better idea what is going on when the shower heater is active. Use a low voltage transformer between the oscilloscope probe and line for isolation.

author: Gyro
author_rank: Super Contributor
timestamp: August 10, 2021, 07:44:13 pm
content: They're more primitive than that. You have a switch which switches in various element combinations in the heating chamber (High, low etc.) and then you control the temperature by varying the water flow with a rotary valve knob. The only contacts, apart from the switch, are the overheat protection thermostat and the low water pressure protection sensor.

author: ejeffrey
author_rank: Super Contributor
timestamp: August 10, 2021, 07:56:59 pm
content: Quote from: David Hess on August 10, 2021, 07:32:52 pmIf the voltage spike from relay contacts switching a 40 A resistive load is blowing fuses on microwave ovens then something is seriously wrong with the house wiring, the shower, or the microwaves. Possibly all three. There certainly could be something wrong with them, and monitoring with a DSO is good idea but there is absolutely no reason that an electric shower needs zero crossing switching.If I had to guess I would say that it is a crappy microwave oven and the correlation with the shower is either coincidence or simply because the microwave is ready to fail at a moment's notice.

author: David Hess
author_rank: Super Contributor
timestamp: August 11, 2021, 03:32:57 am
content: Quote from: ejeffrey on August 10, 2021, 07:56:59 pmOr marginal power *and* crappy microwave ovens.The poster is UK so I assume straight 240. In the US the heater would be on 240 but the microwave over on split phase 120 so a bad neutral could cause this sort of thing.