Commercial dishwasher heater element

[pennychew]

Commercial dishwasher in a site i look after went 'pop' today, it stopped working a couple of months ago and i found a 4a fuse had blown on the PCB. I changed the fuse and it has worked fine until today, took the cover off and saw one of the three heating elements has burnt out.

Found the part in the spares catalogue so i'll order one up and i'll just tidy up the wiring connections but im just wondering about how this has been connected up now i'm looking at the photos on my phone (there was a diagram on the inside of the panel but i didnt take a pic unfortunately) the element has 3 cores and 6 connections, i'm assuming its L1,L2,L3 and the bottom 3 connections are linked together but there doesnt appear to be a return path on the photos. Also someone has disconnected one of the cables and taped it up previously.

I can find out the other stuff tomorrow morning and get the wiring diagram but thought i would get some opinions on the operation and cause of failure if anyone has them!

Stu


From the spares catalogue

 

[Marvo]

Sounds like they're wired in star. The center point of the star where all 3 elements have one side bonded together could be connected to a neutral or it could be floating in which case no wire will be connected to it, just depends on the design.

Remove old element, descale tank and fit new one. If you don't check for scale build-up you might be back again soon when the new element goes bang. Make sure tank is full before heating contactor operates. It's always worth making sure the level controls are working properly and don't permit heaters to operate without water. I'd also commission with the element disconnected to avoid any unhappy outcomes.

 

[pennychew]

Doesnt appear to be connected to a neutral which is what i would expect...

Actually its rated in the catalogue as a 420v element so star makes sense now

 

[pennychew]

So would one leg not connected cause premature failure?

 

[Marvo]

Very unlikely, it would cause very slow heating though. Most common cause of element failure would be dry heating so like I mentioned make sure the controls are working properly, if there's level probes in the tank make sure they're not covered in scale etc.

 

[pennychew]

Yeah there is a float switch on the side of the tank that I checked on one at a different site recently. I'll bell it out when I pass in the morning and get the wiring diagram

 

[Marvo]

Doesnt appear to be connected to a neutral which is what i would expect...

Actually its rated in the catalogue as a 420v element so star makes sense now

A true 420v element assembly would be connected in delta but just follow the wiring diagram and if in doubt rather go with star wired which is the safer option.

 

[pennychew]

Yeah sorry I'm mixing the two up, it's the lack of neutral that has thrown me. I'll just fit the new part when it comes and wire up like for like anyway. £120 for the assembly the robbing barstewards!

 

[Marvo]

Yeah it's not cheap but it's possibly a high grade stainless steel element tube especially if there's chemicals such as surfactants or dispersants like rinse-aid being dosed into the water. The cheaper elements are incaloy or hasteloy but they're not for use in even low chemical concentrations.

 

[telectrix]

never had any trouble with my dishwasher.

 

[pennychew]

Yeah sorry I'm mixing the two up, it's the lack of neutral that has thrown me. I'll just fit the new part when it comes and wire up like for like anyway. £120 for the assembly the robbing barstewards!

More than likely is stainless as all the parts seem to be.

 

[pennychew]

Went in this morning as id had the idea through the night of disconnecting the faulty element so they could use the machine although it would take longer to heat between cycles.

Disconnected the faulty element and removed and cleaned the float switch (it was covered in scale deposits but the boil tank was full of water so don't think its been running dry) it took an age to heat up and then started to cool when it was trying to fill the main tank.

For some reason the drain pump was trying to pump all the time so i spent about 3 hours testing things and looking for the fault but to no avail. Decided to call the service dept and get them out as they can't afford to be without it for too long.

The elements were wired in star but when i clamped the working element it was pulling 7amps on L1 8 amps on L2 and only .5 amps on L3, another reason i decided to back heel this one!

 

[pennychew]

Wiring diagram in case you were interested!

 

[Marvo]

Sounds like one of the tubes on the element is open circuit or there's a phase missing at the elements.

As a rule you should never start to fault find any sort of control system unless you're 100% familiar with what it's controlling. You need to know all the input devices, all the outputs and the components that are being driven or supplied and you need to know the sequence of operations including the relationships between the inputs and outputs. If not you're going to be in for a long and possibly expensive learning curve.

Even just replacing an obviously blown element is risky because there's always a possibility its demise was brought about by a fault elsewhere in the control system.

 

[pennychew]

According to the diagram, the elements are individually fused but there was no obvious fuse betweeen the contactor and the element so it may be built into the element, there was voltage at all 3 phases.

You're second point is a fair one however I do have a lot of leeway with this company as I've fixed all sorts for them at different sites in the past, they are prepared to let me have first crack as it saves them a fortune in service costs, then worst case if I can't fix it they'll bring them in anyway. It's also turned into a lucrative market for me as I'm usually there for other jobs anyway so I can just charge a few extra hours Labour and some mark up on parts.

One of the area managers has just left that company to run a big hotel close to me and I'm now getting a foot in the door with them as he likes the fact I offer that kind of service.

You've just got to be sensible with what you will and won't touch, as a thread in the other forum says 'a man's got to know his own limitations!' but at the end of the day sometimes you've just got to get the job done

 

[happysteve]

Wiring diagram in case you were interested!
View attachment 27491

Is that a fuse in the N (F2)? Is that sort of thing normal? Just interested...

 

[Bobster]

According to the drawing its a 4A fuse, not normal.

 

[happysteve]

Thanks.

I'm trying to think of a situation on an installation, control gear or equipment where a fuse in the N might be needed or justified. I'm stuggling.

I suppose it could protect the upstream (where "up" is "closer to source of energy") cabling, e.g. if there was a possibility of the N downstream of the fuse being in contact with a line conductor that has not been fused down itself. But this seems very convoluted!

There was a thread about fused neutrals a few months back (can't find it), I can't recall the reason why N used to be fused at the service head in the old days (if there ever was one!).

 

[Bobster]

It originated in the days of DC transmission, people at the time saw it as being the safest option (they were proved wrong).

I can't think of any reason off the top of my head why a neutral would be fused, it's quite common to have the 0V/Neutral in a control panel to have a drop link fitting,. This is only to aid in testing.

 

[Marvo]

The only time I can think of for a fused neutral is if there's a chance the supply polarity could be reversed.

Maybe they did it here to limit surge currents through MOV's to earth. It's a curious and very unusual arrangement.