Fuse blown and neutral wire sheathing melted.

[Jimmy Jones 1234]

Hi guys, hope you can try to shed some light on what might be the problem. Basically nothing is new, all been the same for 3 years since we moved in but over the past few days when the dishwasher was on (or anything plugged into a socket in the kitchen) I noticed a burning plastic smell. Couldn't figure out what it was until a fuse tripped the other day (popped actually - I thought a lamp bulb had blown), I opened up the fuse box panel in the gypsum wall and saw the melted plastic fuse box cover, took that off and saw melted plastic on the wires. The fuse on the right is what tripped but it only has the kitchen sockets on it, nothing big on here only a fridge and the dishwasher, from time to time the toaster and kettle. The one in the middle serves the bathroom lights and on the power to the corridor. There aren't any big loads here at all. I can't figure out why the plastic would melt like this. I tested it again and when I plug anything for long term (like the dishwasher) the fuse gets warm and then eventually hot and I'm guessing will trip again.

I will call an electrician obviously, but when the 'Corona virus' is over, not now - I'm staying home and not inviting strangers to my home either. I have kept the fuse off for now and using an extension lead for the appliances from another socket in the next room.

Attached is a picture.

 

[telectrix]

diffivult to tell from the pic. you say the fuse on the right tripeed, but the burn damage looks more like on the left. i suspect a poor connection/termination causing the problem.

 

[Jimmy Jones 1234]

diffivult to tell from the pic. you say the fuse on the right tripeed, but the burn damage looks more like on the left. i suspect a poor connection/termination causing the problem.

Hi, yes the burn damage is where the neutral and earth wires come together on the left. They are all secured tightly in the metal clamp on the left which was originally fixed onto a blue plastic holder. That holder at the back is black and melted too, so guessing the problem has to do with the neutral/ earth wires getting hot which melted the plastic. But yes, the only fuse that tripped was the one on the right.

 

[SparkyChick]

Definitely some better pictures would help, maybe showing where the cables go at the side, top and bottom.

 

[Jimmy Jones 1234]

Okay, couple of extra pics. The cables at the top (sheathed in black) come from the outside fuse box (in the garage) the bottom live cables come from the left the same as the neutral and earth cables which are from the kitchen sockets, bathroom lights and hallway power.

 

[telectrix]

side, top and bottom.

now you're giving things away.
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i suspect the termination/s on the top side of the left breaker.

 

[Jimmy Jones 1234]

side, top and bottom.

now you're giving things away.
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i suspect the termination/s on the top side of the left breaker.

Do you mean they might be loose, need tightening? I'll check.

 

[telectrix]

think it needs more than that. stripping back to good cable and reconnecting.

 

[Lucien Nunes]

it only has the kitchen sockets on it, nothing big on here only a fridge and the dishwasher, from time to time the toaster and kettle

Dishwasher, kettle and to a lesser extent toaster are all large loads. All 3 on together (with dishwasher heating) could total arounhd 28 amps. What is the rating of the circuit breaker?)

The cable insulation to the neutral/earth block seems burnt due to one poor connection overheating and conducting heat to the other cables. If as it appears this is indeed a combined neutral/earth termination it is vitally important that the main connection to the incoming cable on this block is secure. This is a TN-C system (not used in the UK) where one conductor serves as both neutral and earth. If the connection is broken all appliance casings can become live and present a risk of severe shock.

UK readers, also note the likelihood of it being 3-phase as well as TN-C. Estonia has both German and Soviet influences and material.

 

[Jimmy Jones 1234]

think it needs more than that. stripping back to good cable and reconnecting.

Just took the cable out on the top left and had a look, completely shiny, no burn or black, actually like new, put it back and tightened up the screw on the fuse. Checked the other fuses as well and all wires very tightly secured. What I don't understand though is why the neutral wire sheathing and also some of the earth wire sheathing is brittle/ melted. No wires here are loose that I could tell, they all seem secure.

I've put the fuse back on at the moment and going to boil a kettle, put the dishwasher on, burn some toast and see if the fuse gets hot on the right.
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Dishwasher, kettle and to a lesser extent toaster are all large loads. All 3 on together (with dishwasher heating) could total arounhd 28 amps. What is the rating of the circuit breaker?)

The cable insulation to the neutral/earth block seems burnt due to one poor connection overheating and conducting heat to the other cables. If as it appears this is indeed a combined neutral/earth termination it is vitally important that the main connection to the incoming cable on this block is secure. This is a TN-C system (not used in the UK) where one conductor serves as both neutral and earth. If the connection is broken all appliance casings can become live and present a risk of severe shock.

UK readers, also note the likelihood of it being 3-phase as well as TN-C. Estonia has both German and Soviet influences and material.

Thanks. it's not 3 phase. No idea what the fuse rating is as it's so old all the text on the front has worn off. Point is also that this just happened recently - it's a 'new event' if you like, we've been here three years and last week is the first time we smelt the plastic burning smell. I was in this fuse box about a month ago (can't remember why) and there was no burnt plastic or cables then, so it's recent.

Also, this happened (the fuse tripping) when only the dishwasher was running, nothing else. But again, the dishwasher isn't new - everything is the same it has been in 3 years.

I will check in the garage to make sure all the cables are tightly connected coming in to this fuse block.

 

[Lucien Nunes]

No wires here are loose that I could tell, they all seem secure

But if the surfaces of the wires are oxidised where they enter the terminals the connections can still have high resistance, and therefore overheat, despite being mechanically secure. Only cleaning and reterminating to a bright, shiny section of copper will ensure adequately low resistance.

going to boil a kettle, put the dishwasher on, burn some toast and see if the fuse gets hot on the right.

What is its rating in amps?

Another aside for the UK readers, note the colour code of the soviet-era incoming cable: L1 Yellow, L2 Green, L3 Red, PEN Black. Pete999 will probably remember this stuff!

 

[Jimmy Jones 1234]

But if the surfaces of the wires are oxidised where they enter the terminals the connections can still have high resistance, and therefore overheat, despite being mechanically secure. Only cleaning and reterminating to a bright, shiny section of copper will ensure adequately low resistance.


What is its rating in amps?

Another aside for the UK readers, note the colour code of the soviet-era incoming cable: L1 Yellow, L2 Green, L3 Red, PEN Black. Pete999 will probably remember this stuff!

Okay, good idea, I'll clean up all those wires on the left (neutral/ earth) the other wires to the fuses are okay, I checked those. But why would the wires oxidise like this all of a sudden, they were okay, nice and shiny before this 'melt down' so to speak which has now caused the oxidising. Surely then there must be some other fault along the line?

The incoming cable (the 4 cables at the left top and sheathed in black pvc); the neutral wire is blue (it's now black sheathing halfway along because of the burning, the left fuse has a dark green cable sheath, no idea why, the other two to the middle and right fuses are both brown. But mostly the cables in the house are brown, blue and yellow/green. The house was renovated in the mid-nineties luckily after the soviet era but the cables coming into the main fuse box are all sorts of greys, browns, greens etc.

You asked what it's rating is in amps, I have no idea, there is no lettering left on the fuses.
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Also, one thing my wife reminded me is that our coffee capsule maker was the thing that went 'pop' and actually blew when the fuse blew, it's kaput now, but it was plugged into the kitchen socket at the time but actually it wasn't switched on or being used at the time. Although before this I had smelt the plastic burning smell without figuring out what it was and not when the capsule machine was on either. Is it possible, that even though it was only plugged in but not switched on that it could have had a fault that caused the overheating and the fuse to blow? I wouldn't have thought something just plugged in to a socket and not actually switched on could do something like this.

 

[Lucien Nunes]

why would the wires oxidise like this all of a sudden

It can be a self-accelerating process. If there is just a hint of resistance, say through corrosion, then the joint starts to warm up and worsen the oxidation, which increases the resistance, etc. It can take time to develop but eventually goes into runaway and what was apparently a good connection rapidly overheats.

Overheating can also be due to overloaded cables, but then it tends to be the middle of the cable that gets hottest and the ends that are clamped into the terminals are cool because the heat is conducted away from the cable. In your case, the main source of heat seems to be one of the neutral connections, in which case the current is limited by the circuit breaker in the corresponding line and overloading should not occur as the breaker would trip first.

Is it possible, that even though it was only plugged in but not switched on that it could have had a fault that caused the overheating and the fuse to blow?

Not likely. As above, the circuit breaker should have prevented any kind of fault causing any overheating, and even if the circuit breaker failed to trip on a short-circuit, the flex of the coffee machine would likely have been the first thing to melt, not the (larger) installation cable.

Overheated connections are a common problem in electrical maintenance and 95% of the time it's simply oxidation / looseness / whatever source of high resistance that triggers the process. I am not at all convinced that the overheating and the tripping of the breaker are from the same root cause. I think the terminal was overheating already, but you were not really aware of it until the fault that tripped the breaker made the burning smell worse and brought it to your attention.


I can read some text on the breakers. I would not be surprised if they were B16s. Is there really nothing visible?

You say it is not a 3-phase system. If that is the case, the blue PEN conductor of the incoming cable is probably too small and may be overheating. It is the same size as the three lines, which is correct for a 3-phase supply but only a third of the size it needs to be for single-phase, as all the current used in the installation must then flow through it. Can you post a pic of the panel in the garage where it comes from?

 

[Jimmy Jones 1234]

It can be a self-accelerating process. If there is just a hint of resistance, say through corrosion, then the joint starts to warm up and worsen the oxidation, which increases the resistance, etc. It can take time to develop but eventually goes into runaway and what was apparently a good connection rapidly overheats.

Overheating can also be due to overloaded cables, but then it tends to be the middle of the cable that gets hottest and the ends that are clamped into the terminals are cool because the heat is conducted away from the cable. In your case, the main source of heat seems to be one of the neutral connections, in which case the current is limited by the circuit breaker in the corresponding line and overloading should not occur as the breaker would trip first.



Not likely. As above, the circuit breaker should have prevented any kind of fault causing any overheating, and even if the circuit breaker failed to trip on a short-circuit, the flex of the coffee machine would likely have been the first thing to melt, not the (larger) installation cable.

Overheated connections are a common problem in electrical maintenance and 95% of the time it's simply oxidation / looseness / whatever source of high resistance that triggers the process. I am not at all convinced that the overheating and the tripping of the breaker are from the same root cause. I think the terminal was overheating already, but you were not really aware of it until the fault that tripped the breaker made the burning smell worse and brought it to your attention.


I can read some text on the breakers. I would not be surprised if they were B16s. Is there really nothing visible?

You say it is not a 3-phase system. If that is the case, the blue PEN conductor of the incoming cable is probably too small and may be overheating. It is the same size as the three lines, which is correct for a 3-phase supply but only a third of the size it needs to be for single-phase, as all the current used in the installation must then flow through it. Can you post a pic of the panel in the garage where it comes from?

Lots of helpful info, much appreciated. if the fuse trips, could that blow the capsule coffee maker though while plugged in? Glad it didn't blow the fridge

Attached pics, it's really old fashioned - when I make the extension next year I plan on putting in a new panel with modern fuses etc. Hopefully, you can make everything out okay. Right in the middle at the top is the black wire (I presume - it has the same black plastic cover as what is inside the house and is the only one like that).

 

[pc1966]

WTF! Is that an exposed live conductor on the circular fuse (next the the single breaker, top left)?!

It all has a bit of a three phase feel to it though, 3 fuses middle left, three breakers in box top right, what looks like a 4 or 5 pin power socket bottom left...