See if anybody can work this one out...

[jalaner06]

Hey everyone,

Let me know your thoughts on the following:

Three weeks ago I was called out to a cafe when the head chef plugged his microwave into a double socket and it went "bang" and took out the 32amp type B mcb that supplies the ring circuit, upon arrival I checked the microwaves 13amp plug top, there was no sign of damage and a continuity test showed that the 13amp fuse inside the plug top was fine, I energized the ring circuit and plugged the microwave back into the double socket, no problems at all and the microwaves lcd screen lit up fine, I then checked in the back of the double socket for any signs of damage to wiring and there was no sign of damage, I replaced the double socket just incase it was faulty.

I then plugged the microwave back in to the new double socket when energized and heard a massive KaBbad day explodeoooom that came from the plug top, again.... it tripped the 32amp B type MCB at the board, but didn't blow the 13amp fuse???.

I turned the breaker back on, plugged the microwave back into the energized socket, microwave was working totally fine again...?

The chef had a spare microwave (exactly the same model as the faulty one), so I told him to use the spare microwave and leave the faulty one aside whilst I give it some more thought.

The new microwave worked fine for a further two weeks being used daily, but then I got a call to say that the new Microwave had blown the ring circuit, the chef heard a bang at the plug top, I arrived, 13amp fuse still in tact and microwave was working totally fine when re-energized.

I did insulation resistance tests on the ring circuit at the board, all above 99.9Mohms on all tests when all accessories removed from circuit, Earth loop was fine, circuit is totally healthy in every way.

So something about this ring circuit doesn't like this model of microwave, but its seriously bugging me and I cant sleep at night.

The ring main in question is in a Kitchen, there are always 3 fridges also plugged into this ring main, two fans usually on, a motorized under sink grinder, some hot plates to heat meat, and various other things, I also tried plugging the microwave into its double socket when all of the above accessories were also plugged into the ring circuit and in operation, and the microwave still was fine.

Ow... and the ring circuit was pulling a total of 7.4amps when under full load, so not a lot.

Any ideas guys???, or have I been beaten???.

 

[revjames]

MCB will trip before a 13A fuse has time to blow if the fault current is high enough. Sounds like a major perhaps intermittent problem with the microwave if you ask me.

 

[extension15]

I turned the breaker back on, plugged the microwave back into the energized socket, microwave was working totally fine again...?

1 Has any other equipment been used in this particular socket and does it cause a problem?

2 Whose 13A sockets are being used?

3 Has the Microwave been plugged into any other socket?

4 Can you PAT test the Microwave?

:|

 

[bengie]

i'd say the bang came from the socket simply because thats where you making the connection either with the switch or by plugging in the appliance while the switch is on.

does sound like dodgy equipement to me.

 

[jalaner06]

1 Has any other equipment been used in this particular socket and does it cause a problem?2 Whose 13A sockets are being used?

3 Has the Microwave been plugged into any other socket?

4 Can you PAT test the Microwave?

:|

Hiya...

In answer to your questionsw:

1 the hot plate mentioned in my first post is also pluged into this double socket, the same double socket as the microwaves have been, the hot plate does give off a heap load of heat but is about 1 metre in distance from both the double socket and the microwave, I would say the heat given from the hot plates is a factor to consider.

2 crabtree 13amp double socket

3 dont forget this has happened with two different microwaves of the same model, i had briefly pluged one of the microwaves into a different ring circuit in the building, it worked fine, but it hasnt been used over a long period of time in a different ring circuit.

4 I havent PAT tested the microwave, I sadly dont have one to hand, its a class one microwave so it is totally made of metal.

I had to replace a 10amp fast action HRC fuse in one of these microwaves once about a year ago, they have a small electromagnetic fan inside that cools a small transformer inside, I dont know if any of this is of any use at all?.

i'd say the bang came from the socket simply because thats where you making the connection.

I agree, I thought this also so didnt look to much into the origin of the "bang" itself.

 

[extension15]

OK, if no other equipment causes an issue and the circuit etc ifs fine.............

And both Microwaves are the same model...............

 

[extension15]

Hiya...In answer to your questionsw:

1 the hot plate mentioned in my first post is also pluged into this double socket, the same double socket as the microwaves have been, the hot plate does give off a heap load of heat but is about 1 metre in distance from both the double socket and the microwave, I would say the heat given from the hot plates is a factor to consider.

2 crabtree 13amp double socket

3 dont forget this has happened with two different microwaves of the same model, i had briefly pluged one of the microwaves into a different ring circuit in the building, it worked fine, but it hasnt been used over a long period of time in a different ring circuit.

4 I havent PAT tested the microwave, I sadly dont have one to hand, its a class one microwave so it is totally made of metal.

I had to replace a 10amp fast action HRC fuse in one of these microwaves once about a year ago, they have a small electromagnetic fan inside that cools a small transformer inside, I dont know if any of this is of any use at all?.

i'd say the bang came from the socket simply because thats where you making the connection

I agree, I thought this also so didnt look to much into the origin of the "bang" itself.

Try a different make of Microwave?

It's not uncommon to have a faulty batch of equipment

 

[jalaner06]

Try a different make of Microwave?

Well thats what I thought to be honest, the chef aint going to be happy with me, these microwaves go for a good

 

[extension15]

Well thats what I thought to be honest, the chef aint going to be happy with me, these microwaves go for a good

 

[steptoe]

have you confirmed polarity at the socket?

ie, if you are getting your line throught he neutral connection then a fault to earth will NEVER blow the fuse.

just a thought.

 

[SPECIAL LOCATION]

And have all of the other tests been done to verify the integrity of the circuit?

Although it may sound daft..

with these sort of problems you do need to verify the basic ground rules of everything..

e.g.

L-L ring continuity

N-N ring continuity

E-E ring continuity

& verify all appropriate for cable lengths

Insulation Resistance.

etc..

Do the microwaves work alright if plugged into a different ring circuit?

 

[Theorysparky]

slightly off topic....

why is a chef using a microwave ????

 

[SPECIAL LOCATION]

slightly off topic....why is a chef using a microwave ????

5-day wonder Chef!!!!!!!!!!!!!!!!!!!!!!!!

:^O :^OROTFWLROTFWLROTFWL



I only paid a grand and within a week i wuz qualified! ]

 

[vandalay]

Hiya...In answer to your questionsw:

1 the hot plate mentioned in my first post is also pluged into this double socket, the same double socket as the microwaves have been, the hot plate does give off a heap load of heat but is about 1 metre in distance from both the double socket and the microwave, I would say the heat given from the hot plates is a factor to consider.

.

Could the hot plate be giving out enough heat, if it was close , to affect the cable,as in after a while heating it up so the MCB registers a overload ?

 

[green-hornet]

The idea that it goes with a bang as opposed to a simple disconnection, would rule out overloading of the circuit.

Any bang would suggest a problem that needs to be sorted.

Besides any end to end tests I would be inclined to test from socket to socket,if no rcd is fitted which I would expect in an industrial kitchen, there could be a problem of low insulation on one leg only, and if corrected your IR reading could realisticly be above 200 ohms

 

[Mr Smith]

although not eleectrically trained

mr smith wonders if moisture

on the face of the socket

could cause a problem such as this

mr smith

 

[ProDave]

4 I havent PAT tested the microwave, I sadly dont have one to hand, its a class one microwave so it is totally made of metal.

You can "pat test" the microwave just using your normal tester. All a PAT tester does is wrap up the same tester in a convenient box with a 13A socket to plug the appliance into.

You need to strap L-N on the plug and test from there to E for insulation with your insulation tester. Then test from the earth pin of the plug to some part of the metal case for earth continuity.

 

[Lurch]

What microwave is it? Some of the commercial ones are quite power hungry. Might be a cap on the mains just taking a whack to charge when it's plugged in. Adding a couple of radials for the microwaves can sometimes be handy. Usually I'll try and wore commercial kitchens with as many radials for all equipment that is known with a ring just for GP sockets. With the amount of motors and surges a ring for everything can sometimes be a bit problematic.