Take, for example, insulated neutral transformer secondary. Protection against over voltage in this case is afforded by MOV's discs connected between each phase and ground. When a high voltage appears (such as insulation breakdown between HV and LV) on the secondary conductor , the MOV breaks down to 'ground' the affected phase. Just a self-sacrificial duty in this case!
exploding surge protector
- Thread starter petersonra
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weressl
Esteemed Member
What a bunch of nonsense! What you describe here is never done for the purpose you claim to. MOV's are never expected and applied for a one-shot deal.
What does the "protected" indicator mean? If the indicator is not lit does that mean that there is no surge protection?
If the surge is high enough can it destroy an MOV in one shot?
As I understand MOVs they are extremely high resistance which only start to conduct when a given voltage is reached at which point they could be destroyed.
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AFAIK, the "protected" indicator measures that a small amount of current is flowing through the varistor under normal voltage. Just enough to show that is has not been open circuited as the result of fault current.
And yes, if the surge is small enough (in both voltage and current) the MOV will have enough thermal mass to absorb/clamp the surge without damage to itself.
If the surge is larger, the MOV will either fail by shorting (which would be apparent and an OCPD somewhere would open) or by open circuiting, in which case the protected indicator would go out.
But if you know that any surge arrestor has been subjected to a major surge, it would be conservative to replace it, since you do not have the tools to test it fully.
eHunter
Senior Member
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The "protected" LED is there to indicate that the internal OCPD is intact and is deceptive about the condition of the MOV(s).
In most that I have seen, the OCPD is a glass pigtailed fuse soldered into the PCB on the line side.
When the MOV experiences a large enough surge the smaller disc MOV units will usually simply separate opening that part of the circuit without blowing the fuse and the LED continues to illuminate even though the MOV is damaged and is of no use.
When the surge is even larger the smaller disc MOV units will usually not open quick enough to prvent the fuse from opening and the LED will no longer illuminate.
http://www.zerosurge.com/technical-info/truth-about-movs/
No it is not. For protection of loads on transformer secondary side with insulated neutral against sustained over voltages such as caused by insulation breakdown between HV and LV, the method mentioned in my post #21 had already been practically applied. But if you know any other method comelier for the same purpose, let us know.
weressl
Esteemed Member
So let me understand how this supposed to work.
There is a breakdown between the primary and secondary side of transformer winding. High voltage occurs on the secondary side of the transformer. MOV conducts. Keeps conducting until it eventually fails and it fails open. Then WHAT?! The high voltage is still present on the secondary side. Lightning and surge - hence their name - protectors are designed to protect against transient overvoltages not sustained overvoltage. Transient means: of limited time duration.
Fails open? Not fails grounded? Evidence please.
weressl
Esteemed Member
In an unlimited current condition the MOV will first fail to a low impedance (few Ohms), but due to the high amount of energy available, it most often ruptures instantaneously. If equipped with thermal protection according to UL1449, the thermal protector is required to open the circuit before the MOV can disintegrate.
It is an ungrounded system both upstream and downstream to a transformer, the ground fault current would not be high and there is no chance of fault current turning from 'A' to 'kA' to rupture/explode the arrester and so the arrester should stay grounded under such fault conditions.
weressl
Esteemed Member
I2t
If the system is ungrounded on both upstream and downstream sides of the transformer, the voltage to ground is indeterminate. The whole system is floating. Even with a primary to secondary fault, it would still be floating. Indeterminate WRT ground.
In such a case, it is nonsensical to suggest, as you did in post #21, that "protection against over voltage in this case is afforded by MOV's discs connected between each phase and ground"
Mods, don't worry. I'm not going argue the point. It's just too silly to bother with.
Even ungrounded system has nominal voltage to ground value defined for it. Capacitive effect may increase (not decrease) its actual value, thus making it indeterminate. But this is not relevant to the discussion here.
Anyway please do not approach and touch any bare ungrounded system wire on the assumption that its voltage is indeterminate!
Yes, indeterminate WRT ground till the subject MOV breaks down and conducts through its ground lead. Then the ungrounded system faulted line HV becomes determinate and equals practically zero with respect to ground.
If you wish to discuss this nonsense further, please do so by PM.
See my PM. You did not understand at all.
I received no PM from you.
You should have had. Check your inbox again.
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