When does micc short or go bang

[Stevie g]

Micc readings on a 4 core 10 mm are 4 cores to earth 1.09 ohms and between cores 1.70 ohms tested at 500volts cable supplies main DB and off peak DB both backed up by 60 A bs88 fuse, what's the resistance reading likely to be before it goes bang?

 

[MJPD29]

A lot lower i would think

 

[westward10]

Do you mean meg ohm and not ohm. As it stands your 1.09 ohm would equate to over 200A.

 

[littlespark]

Is this with all loads disconnected? 1.09 ohm is very low. should be Mohm if testing IR.

Beaten to it

 

[DPG]

Shows how important it is to get the right units. Can cause much confusion otherwise.

 

[Lucien Nunes]

If you mean megohms....

The highest voltage within the cable seems to be 230V (if main and off-peak supplies are on the same phase). Worst case leakage is then going to be from two line cores to two neutral cores and to the sheath.
1.7||1.7||2x1.1 = 0.6MΩ
230/0.6M = 0.4mA = total leakage from both lines
230²/0.6M = 88mW = power dissipation in leakage resistance

So it's not going to go bang like that. It would probably need to be between 10 and 100 times lower to get into a runaway situation, although at 6kΩ the cable sheath would probably get warm enough to drive off the moisture if it didn't track and flash over. What you can't tell, though, is whether that IR value is because of a tiny leak that has been like that for decades, or a recent damage that is going to get noticeably worse. You can sometimes localise the problem by warming the seals and any likely point of damage with a heat gun; when you get to the damp bit the reading will duck and dive as you heat.

OTOH, if you mean ohms....

It already went bang.

 

[telectrix]

if he means ohms, it's already gone bang.

 

[littlespark]

if he means ohms, it's already gone bang.

You're a bit late there, scouser Tel... @Lucien Nunes already used that line.

 

[telectrix]

You're a bit late there, scouser Tel... @Lucien Nunes already used that line.

oh well, never the bride, always the bridesmaid.

 

[Lucien Nunes]

Tel, they say great minds think alike, I just happened to think alike first.

 

[Andy78]

Thinking back to college, MI doesn't go bang until you flatten it out to three times its original width on an anvil.

 

[DPG]

Thinking back to college, MI doesn't go bang until you flatten it out to three times its original width on an anvil.

And YouTube probably

 

[freddo]

You're not getting misleading readings from a sub meter on the supply?! Try the heat test mentioned above just behind each seal, easier to see with an analogue insulation meter.

 

[ipf]

I'd like to think the fuse would pop before the cable went bang!

 

[KennyKen]

I’ve had a few MICC’s Blow themselves Clear. ?

 

[Stevie g]

Sorry my bad ,yes Megohms, cable is 40 meters and around 80 years old, old imperial micc,
Thank you all for response
[automerge]1580844854[/automerge]

You're not getting misleading readings from a sub meter on the supply?! Try the heat test mentioned above just behind each seal, easier to see with an analogue insulation meter.

No nothing in between kmf and DB, meters before kmf, analogue do they still make them only joking
Thanks it was periodic, so cross that bridge if we get the repairs

 

[freddo]

As long as the fault is damp in one of the ends (and not pinholes in the sheath) and the sheath hasn't corroded badly, and you can access the ends for repair, it should be repairable. MI cable is amazing stuff.