I have posted this question on picoauto forum, but I have no answer for two days. I want to work so maybe you can help me clarify this.

I have just purchased a standard kit, which has 4 attenuators 1:20. There is a warning on each of them "max voltage 300V". One of main uses of these attenuators would be ignition primary, which can easily have spikes up to 400V. I am worried not to cook the attenuators measuring this ignition primary, and in case this may happen - cooking the scope input due to over-voltage (giving the direct 400 V spike to scope instead of 20 which the attenuator would produce). On the other hand, I've seen so much manuals from different sources, with people using these attenuators for ignition - without worrying to damage anything.
I would appreciate your comments on this case. Thank you !

Hello Partyzan!
I looked at the information on the site here where you can buy them, they do list as Max 300V input. The scope itself has a 100V max input, so the attenuators are basically there to de-spike, they handle the ignition spikes (pri) with no problem.
If you're still concerned send Tom a message, he can
likely explain it a little more elegantly than this.
On a hunch the may be rated 300V continuous, more on spikes. I do know if they didn't work on ignition primary my scope would have been in for repair many many times now.
-Ward-

Partyzan wrote on Feb 21^st, 2014 at 4:16am:

I am worried not to cook the attenuators measuring this ignition primary

You have no worries there.  Just be sure that the scope is always grounded to B neg at all times.  The attenuators need to be grounded to work properly.  The scope grounds are common so only one channel need be grounded.

I see, thank you.

In relation to Tom's last post, I came to another problem. Tom said that all channels have the same ground.
In this case I want to ask: if we connect one channel correctly (scope ground to battery "-" and scope signal to battery "+"), then we connect second channel ground to anything with the voltage - this way we are in the risk of burning scope or vehicle device, is this correct?

This came to my head after I remembered that some signals have to be measured not on base ground, but to the signal ground - which may be a few volts above base ground (e.g. some lambda sensors have risen ground of about 2 volts).  Or on the worst case if some wires gets mixed and second channel ground gets 12 volts B+. If all channels really have common ground, then any of my mentioned situations are surely cooking the scope and smth. in the car ... Aren't they ?

Partyzan wrote on Feb 26^th, 2014 at 1:12pm:

If all channels really have common ground, then any of my mentioned situations are surely cooking the scope and smth. in the car ... Aren't they ?

Not likely.  The automotive PicoScope models have resetting thermal fuses to prevent damage should a high current ground loop be created.

However, you can cause some interesting things to happen on the vehicle by crossing up your signals by placing channel grounds at different locations.  For example, placing signal and ground across a crank sensor and then another channel across the battery would ground out your CKP.  You can place a channel signal and ground across a sensor but then you cannot use any other channels.

The proper way is to place any and all grounds at the bat neg, if possible.  There are exceptions, but you better know what you are doing and have a good reason.

For sensors with floating grounds, two channels can be used and math channels employed to see the signal as it is while keeping the scope properly grounded.

I share this in the public area since it is a safety tip:

http://www.autonerdz.com/cgi/yabb2/YaBB.pl?num=1285781168/0#0

This couldn't have been explained better . Thank you so much !

PS - I measured the resistance between scope grounds - about 2.5 ohms, so having voltage between them would sure initiate high current.