Exploding BT socket and a lucky escape

[jamieled]

Openreach came out today to sort the master socket. Whatever happened, it melted a load of armoured bt cable and joints and popped stuff in the exchange ~2km away.

[joe90]

In a previous life when I worked fir BT, one of my specialisms was BT kit near generating systems or high voltage kit something called “rise of earth potential” meant we had to protect our network as the earth potential could rise if a high voltage system went faulty and actually cause damage to people or kit at the exchange or joints along the way. Basically “line isolators” had to be installed but so many people in the electrical business had not heard of it and took some convincing it was needed . Might of worked in this instant (but not required) .

[jamieled]

I was asking sse and the openreach engineer if there was anything we could do to stop this happening again. SSE have at least put some insulation on their transformer which might stop contact from any other unlucky flying birds. I don't think there's anything else we can do in the house and the openreach guy reckoned it was just pretty unlucky and that given the large number of rural houses fed from a transformer it's not worth trying to protect their lines from an unusual voltage spike on the power network.

[PeterW]

1 hour ago, jamieled said:

don't think there's anything else we can do in the house

You can fit whole house surge protectors in domestic CUs - they aren’t that expensive at ~£120 or so. Have to get the right one for the installation though as they vary by supply type. 

[wozza]

13 hours ago, PeterW said:

You can fit whole house surge protectors in domestic CUs - they aren’t that expensive at ~£120 or so. Have to get the right one for the installation though as they vary by supply type. 

 

My new CU (Hager Design 50) has one fitted ?

[markocosic]

A theory:

 

 

Cheap and nasty products can use "RC Droppers" as power supplies. 

 

At 0 Hz (DC) the capacitor has "infinite resistance" (won't pass any current)

At 50Hz the the capacitor has "some resistance" (passes "some" current)

At high frequencies he capacitor has "no resistance" (passes all current)

 

 

An electrical arc contains lots of high frequency energy (nt just 50 Hz components)

If you trip the breaker this high frequency arc energy (on the house side) has to go somewhere.

It can go "straight through" your "RC Dropper" power supplies and blows up the connected equipment.

 

 

So the arc energy comes through the mains and is dissipated by the equipment that draws the most current at high frequencies.

 

Perhaps the telephone exchange was that connected equipment? (via your cordless phone power base station)

 

 

I once had a loose neutral* that violently exploded the mains smoke detectors of all things for this reason. (diagnosed by a friend that was a better electrical engineer than I)

 

*Taylor Wimpey build...bent the (through wall) meter tails into the main switch in the consumer unit...tightened the screws...but the meter tails were BEHIND the clamps so they clamped fresh air...worked fine unless you bashed the lid of the consumer unit hard enough to rattle the meter tails by putting something on the shelf next to it.