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Mains Overvoltage Pulse Protection

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spec

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Hello all.

One of our new members had most of his electrical equipment wiped out by a huge mains voltage surge. Also, one of our neighbors had a similar disater when lightning struck the power lines near to his house:
http://www.electro-tech-online.com/...72t-transformer-problems.149422/#post-1278602
http://www.electro-tech-online.com/...ransformer-problem.149421/page-2#post-1278569
http://www.electro-tech-online.com/...ty-pcb-transformer.149401/page-2#post-1278494

On a much lesser scale, we used to regularly loose routers, phones, and Sky boxes (pay TV) when there was lightning in the area, until I fitted suppressed extension leads. Since then there have been no fatalities.

UPDATE: added information: I see the protection as two pronged: local and central.
(1) Local is the protection built into the equipment.
(2) Central is a high-power suppression box that connects where the mains supply enters your property. I was thinking of a device that is protected by some form of voltage catching device for short duration over voltage pulses but that also blew a fuse of tripped a relay for any longer pulse.

While I have an idea about mains surge protectors, particularly local, I was wondering if anyone could advise on a do-it yourself design, or an off-the-shelf unit for central protection. If there were a standard device that didn't cost the earth, that would be great.

Your contributions would be much appreciated.

spec
 
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JLNY

Active Member
At the component level, the devices used in most surge protected circuits are called metal oxide varistors, or MOVs. When the voltage across a MOV reaches above a certain threshold, the MOV begins to conduct, effectively shunting the voltage spike away long enough to protect the device. Generally they can only take so many hits before they wear out, but they are generally a very affordable option. Some devices have MOVs built into the power supply itself, but even so, I typically use surge-protected power strips on any electronics I consider valuable.
 

schmitt trigger

Well-Known Member
Having been involved a very demanding project which required bullet-proof surge protection; this is best done in three steps:

1) Use some series impedance like inductors, PTC thermistors, even a plain resistor. No shunt suppressor can survive a major transient without some series impedance.
2) Use a combination of shunt devices. MOVs, Tranzorbs, spark gaps have all different performance and triggering characteristics. Littelfuse has a good app note explaining those.
3) Increase your creepeage and clearance distance between the power (live) side and the control side. That means not only physical separation of traces and components, but also employing slots on the PWB.


Having said this, your protection scheme is worth squat if you don't have a very low impedance ground path.
That is the reason sensitive industrial equipment require a ground stake or similar at a very short distance from the equipment itself.
 
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AnalogKid

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Generally they can only take so many hits before they wear out, but they are generally a very affordable option.
Both true. But...

For more money and better performance, go with Tranzorbs (GI trade name). This is a high power zener diode built specifically for gigantic-but-short current surges. AC versions are back-to-back zeners in one package. While all transient suppression device performance decreases with age and number of hits, Tranzorbs are way better than MOVs. Here in MIL power supply land, they are the standard. At home, I have upgraded many "protected" outlet strips. The leads on the higher-power parts are stiff and difficult to wrangle into tight spaces, but by definition security is not convenient.

ak
 

JLNY

Active Member
For more money and better performance, go with Tranzorbs (GI trade name). This is a high power zener diode built specifically for gigantic-but-short current surges.
This is another good option. I do not claim to have much experience in this field, and certainly nothing with designing MIL-grade PSUs. I believe Tranzorbs are more generically referred to as TVS diodes (Transient Voltage Suppression) and are used in a lot of the same applications. Most off-the-shelf surge protectors will use MOVs, but TVS diodes may be a more robust option in a DIY solution. Spark gaps-- also called GDT arrestors (Gas Discharge Tube) are also an interesting option.
 
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Nigel Goodwin

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On a much lesser scale, we used to regularly loose routers, phones, and Sky boxes (pay TV) when there was lightning in the area, until I fitted suppressed extension leads.
All of those normally get killed by lightning up the phone line - rather than via the mains.

Do you have protection on the phone line?.

Phone lines are particularly prone, as they commonly use overhead wires - where most mains in the UK is underground and less exposed.
 

spec

Well-Known Member
Most Helpful Member
Many thanks to you all for the information. I did not know about the difference between varistors transorbs. I have added a bit to the the original post because I forgot to complete what I was asking in the OP due to domestic pressure.:D

spec
 
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spec

Well-Known Member
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All of those normally get killed by lightning up the phone line - rather than via the mains.

Do you have protection on the phone line?.

Phone lines are particularly prone, as they commonly use overhead wires - where most mains in the UK is underground and less exposed.
Yes, pulses on the phone lines can blow you equipment. All of the post office sockets I have come across in the UK have quite large voltage suppressors already built-in and, since I fitted mains suppressed extension sockets, the equipment has survived. But I do take your point.

spec
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Yes, pulses on the phone lines can blow you equipment. All of the post office sockets I have come across in the UK have quite large voltage suppressors already built-in and
It's not enough, as I said it's VERY common for lightning up the phone line to damage equipment.

As far as your Sky box goes, remove the phone line from it, it's not used any more - and never really was anyway.

The only thing it was used for was multiroom, to prove your boxes were all in the same house, but new boxes don't even have phone sockets any more.
 
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