Introduction: Two insulation numbers sit side by side in the YC602 specification, and each one protects a different part of an industrial control circuit.
A beginner looking at a 100A latching relay usually reads the contact rating first, because that number decides whether the relay can carry the load. The insulation figures a few lines lower look similar to each other and often get skimmed. In practice they answer a different question: can a fault on the power side reach the PLC, the sensor wiring, or the low-voltage board that runs the panel? The YC602 lists 4000VAC between coil and contacts and 1800VAC across open contacts. They look like two versions of one rating. They are two separate jobs.
Inside a switching relay, the coil side and the contact side share one housing but live in different electrical worlds. The coil is driven by a short DC pulse — 6V to 48V on the YC602 — from a driver board, an MCU output, or a PLC module. The contacts sit on the load line, rated 100A at 250VAC with a maximum switching voltage of 400VAC or 110VDC. A few millimetres of molded insulation separate them. The 4000VAC coil-to-contact rating describes how much voltage that barrier withstands in a one-minute, 50/60Hz dielectric test before breakdown. On a real panel, that barrier is what keeps a load-side event from becoming a control-side problem. Capacitor switching, a lightning-induced transient on the feeder, or a short circuit on the motor branch all push voltage toward the relay's internal structure. If the coil-to-contact insulation holds, the energy stays on the power side. If it gives way, the fault path runs into the driver stage and back through the PLC output card. Swapping a relay is routine maintenance; replacing a controller card is a shutdown.
Separation is about more than surviving a fault. It is also about running two very different signal environments in one device. The coil circuit is a low-energy DC loop with a flyback diode and a driver transistor. The contact circuit carries 100A and produces arcs, inrush current, and fast voltage transitions every time it opens or closes. A thick insulation barrier between them also cuts capacitive coupling, so switching noise on the load side stays out of the control wiring and does not disturb PLC input readings. The 4000VAC figure is the clearest single indicator of how much separation a relay provides. A device with a much lower coil-to-contact rating can still switch 100A, but it leaves less room between the two sides.
The second rating covers a different space: the air gap between the contacts when the relay is open. That gap has one job — stop current from jumping back across after the contacts part. When a 100A circuit opens, an arc forms for a fraction of a millisecond. It dies out as the gap widens and the voltage across it falls below what the air can sustain. The 1800VAC contact-gap rating describes the voltage that open gap holds off in a one-minute test. The number is smaller than 4000VAC because the gap is smaller and the surrounding geometry is different, and it answers a narrower question than the coil barrier does.
Beginners often assume the two numbers should match, or that the lower one is a compromise. The difference comes from what is being tested. Coil-to-contact testing applies voltage between the coil terminals and the contact terminals. The barrier in that test is the molded body, the creepage distance along the surface, and the internal spacing between the coil assembly and the contact assembly. Open-contact testing applies voltage across the two contact terminals. There the barrier is the air gap plus the insulating base that holds both contact arms. Different physical paths and different voltage distributions produce different withstand levels, and relay standards such as UL 61810-1 keep them as separate tests for that reason. What matters for a panel designer is the duty each gap performs. The 1800VAC rating tells you what the open relay holds off between the two sides of the circuit it just disconnected. In a motor branch or a capacitor bank, that means residual voltage stays on the load side while the relay is open, instead of leaking back toward the supply. During maintenance, it means a locked-out relay provides a genuine insulation step rather than a purely mechanical gap. The number is lower than the coil rating, but the job is different: it separates two parts of the same power circuit rather than a power circuit from a control circuit. IEC 60079-19 and IEEE C37.90 both treat insulation and withstand testing as defined test positions, which is why relay datasheets carry the two figures independently.
A designer working on a control cabinet meets these numbers at a specific moment: deciding which relay drives a 100A load and which terminals the PLC actually touches. The coil terminals belong to the control side. They connect to a 24V DC rail or a pulsed driver, and that wiring usually runs in the same duct as sensor cables and analog signal pairs. The contact terminals belong to the power side. They sit next to busbars, capacitor banks, and motor feeds. The relay is where those two wiring worlds meet, and the insulation ratings mark the boundary between them. In practice, a 4000VAC coil-to-contact rating gives a designer enough separation to treat the coil circuit as low voltage even when the contact circuit runs at 250VAC continuously and up to 400VAC or 110VDC during switching. That supports the way panels are normally built: separate wire colours, separate terminal blocks, and separate protection for control and power. The 1800VAC contact-gap rating covers the power side, where the open relay must hold off the voltage that remains between two energised parts of the same circuit. The ≥100MΩ insulation resistance value at 500VDC addresses a different aspect again — it describes how little leakage current crosses the insulation under normal conditions, which matters for sensitive measurement circuits and for the long-term steadiness of the barrier. Swapping in a relay with a similar contact rating but lower insulation values changes how much margin a panel has. A 100A magnetic latching relay that meets a lower coil-to-contact number may still switch the same load, but a power-side event has a shorter path to the control electronics. When comparing high current relay options, the coil-to-contact and open-contact figures are worth reading before the price list. The YC602 lists 4000VAC coil-to-contact, 1800VAC open-contact, and ≥100MΩ at 500VDC alongside its 100A 250VAC contact rating, 1A and 1B contact forms, and -40°C to +85°C operating range. Those values describe the isolation duty the relay is built for, while certificates and approvals are documented separately.
The 4000VAC and 1800VAC figures are not competing numbers. One guards the boundary between the control circuit and the power circuit; the other guards the boundary between two halves of the power circuit while the relay is open. A beginner who can tell those two duties apart reads a relay datasheet much faster and spots when an option offers less isolation margin than the panel needs. The YC602 100A latching relay from YongNeng Relay Manufacturer lists both values, which makes it straightforward to check a design's control-side and power-side requirements against one datasheet.
A：They are measured at different test positions. Coil-to-contact testing applies voltage between the coil terminals and the contact terminals, which checks the main insulation barrier inside the relay body. Contact-gap testing applies voltage across two open contacts, which checks the air gap and the base holding both contact arms. The YC602 lists 4000VAC for the first position and 1800VAC for the second, each tested at 50/60Hz for one minute.
A：Because the coil terminals are wired to the control side. If the barrier between coil and contacts breaks down, a power-side surge has a direct path into the driver stage, the PLC output card, or the low-voltage board. A high coil-to-contact rating keeps that fault energy on the power side and protects the electronics running the panel.
A：It means the coil-to-contact insulation withstands a 4000VAC, 50/60Hz test applied for one minute without breakdown. For a panel designer, that is a wide separation between a low-voltage coil circuit and a 250VAC, 100A contact circuit that can see up to 400VAC or 110VDC during switching.
