TITLE: Wait time to grid network disconnection with failing edges
QUESTION [7 upvotes]: Let $G_n$ be an $n \times n$ planar toroidal grid graph, with each node
connected to its four neighbors, with the top row connected to the bottom,
and the right column connected to the left.
Suppose that, within each unit of time, each edge "breaks" for a
randomly placed time interval lasting
$\delta \lt 1$; perhaps $\delta \ll 1$ could be assumed.
The edge-failures are independent of one another.
Let $G_n(t)$ be the graph at time $t$, with, in general, several edges missing.

Q. I would like to know the expected duration $T$ that
  $G_n(t)$ will remain connected for all $t < T$, i.e.,
  the wait time to the first disconnection of one or more nodes
  from the remainder.

Although disconnection is likely to be dominated by the
probability of single-node isolation,
it seems difficult to capture all the different ways that $G_n$ could
become disconnected. Below, the three shaded nodes are disconnected
by $8$ missing edges.

 
 
 

One could of course ask the same question for grid graphs based on $\mathbb{Z}^d$
rather than $\mathbb{Z}^2$ as above.
It seems likely this has been studied for network robustness. Thanks!

REPLY [5 votes]: This is an approximation of the answer. The main message is that, indeed, the probability of disconnection is dominated by isolated vertices.
I will assume that $\delta=1/m$, for some large positive integer $m$. I will also assume a discrete model, where in each basic interval $[i/m, (i+1)/m]$, every edge breaks independently with probability $p=\delta=1/m$. 
Now I estimate the probability $q$ that at one basic interval, the graph gets disconnected. 
Edit: According to the comments below, a disconnected subgraph of the torus grid has a component with at most two boundary components. Moreover, the boundary is either a contractible curve, in which case the component is shaped like a polyomino, or, the boundary consist of two parallel noncontractible curves, each of lenght at least $n$.
For every $k$, there are at most $3^k$ fixed polyominoes with perimeter of length $k$, and each can be positioned in $n^2$ ways. Also, every polyomino has perimeter of length at least $4$. 
Similarly, up to translation, there are at most $3^k$ noncontractible curves of length $k$, and a pair of such curves may be placed in at most $n^4$ different ways.
Therefore, 
$$q\le n^2\sum_{k=4}^{2n^2}3^kp^k + n^4\sum_{k=2n}^{2n^2}3^kp^k \le n^2 (3p)^4/(1-3p) + n^4 (3p)^{2n}/(1-3p).$$
If we assume that $p=o(1)$, then for $n$ sufficiently large, we have $q\le n^2 \cdot 82p^4$; the contribution of noncontractible separators is exponentially small.
Now we may estimate the expected time when the graph is connected as follows:
$$T\ge\frac{1}{m}\sum_{i=1}^{\infty}i(1-q)^i=p\frac{1-q}{q^2}\ge\frac{1-q}{82^2n^4p^7}.$$
If $q$ is very small, that is, if $p\le c/n^2$ for a very small constant $c$, then we may write $$T\ge\frac{1}{6725n^4p^7}\ge\frac{n^{10}}{6725c^7}.$$
For the lower bound on $q$, we may consider $n^2/2$ independent vertices (if $n$ is even). Each gets separated with probability $p^4$, so 
$$q\ge 1-(1-p^4)^{n^2/2}\ge 1-e^{-n^2p^4/2}\ge n^2p^4/4$$
for $n$ sufficiently large.
If we still assume that $p\le c/n^2$, then
$$T\le\frac{p}{q^2}\le\frac{16}{n^4p^7}\le \frac{16n^{10}}{c^7}.$$