There are no practical relays out there with contacts rated for 100VDC. If a an amp fails and puts out 80-100VDC it is very possible the relay contacts will arc and weld. The output relay is chosen with a "form C" contact. A form C has a normally open (N.O.) and a normally closed (N.C.). Normal is defined as shelf state/de-energized. When the relay circuit trips the relay coil is de-energized. The N.O. contact disconnects the speaker from the amp. The N.C. contact closes across the speaker acting as a crowbar (short) across the speaker which adds additional speaker protection.
Summary: If you read the resistance across the speaker terminals when the amp is turned off you'll read zero ohms. To actually read zero ohms the meter lead resistance has to be eliminated. If using a Fluke 87 or similar this is accomplished with the "Rel" function.
Rod Elliott has a great article on detector circuits and a separate one for relays. http://sound.westhost.com/articles/relays.htm. I think that this one may be fairly new as I don't recall seeing it when developing the P/l relay circuits. Although lengthy, it is a must read if you want to know anything about a relay.
The output relay protection in most amps and receivers only uses a normally open contact. Consequently speakers are often damaged despite having an output relay. I repaired a Marantz 140 a couple years ago that had the output relay contacts welded. Unfortunately I can't find the picture I took of the molten contact.
Summary: If you read the resistance across the speaker terminals when the amp is turned off you'll read zero ohms. To actually read zero ohms the meter lead resistance has to be eliminated. If using a Fluke 87 or similar this is accomplished with the "Rel" function.
Rod Elliott has a great article on detector circuits and a separate one for relays. http://sound.westhost.com/articles/relays.htm. I think that this one may be fairly new as I don't recall seeing it when developing the P/l relay circuits. Although lengthy, it is a must read if you want to know anything about a relay.
The output relay protection in most amps and receivers only uses a normally open contact. Consequently speakers are often damaged despite having an output relay. I repaired a Marantz 140 a couple years ago that had the output relay contacts welded. Unfortunately I can't find the picture I took of the molten contact.